First, I cannot add links to this post, so I will attempt to share them in some other way as a reference. So I will be copy-pasting relevant combat log data into this post directly.
Something that I know has been submitted as a bug in the past is that the Focused Attacks Talent only procs from Main-Hand hits of Fan of Knives. If you have a Felstriker (UBRS Dagger from Rend Blackhand) this is pretty easy to test. Go stand in Org between 4x Target Dummies, with Felstriker in the OH. Cast Fan of Knives at 100 energy between the 4 target dummies until you get a felstriker proc in your combat log, and then immediately cast Fan of Knives again. Your combat log will show 8 crits, but only 4 procs of focused attacks. My own Combat Log here shows this clearly.
The Relevant section of that log is here.
01:13.946 Asceric gains Felstriker from Asceric
01:13.946 Asceric gains Lightning Speed from Asceric
01:13.946 Asceric casts Fan of Knives
01:13.946 Asceric Fan of Knives Master’s Training Dummy 1 1 (O: 347)
01:13.946 Asceric Fan of Knives Master’s Training Dummy 2 1 (O: 337)
01:13.946 Asceric Fan of Knives Master’s Training Dummy 3 1 (O: 335)
01:13.946 Asceric Fan of Knives Master’s Training Dummy 4 1 (O: 332)
01:13.946 Asceric Fan of Knives Master’s Training Dummy 1 1 (O: 1404)
01:13.946 Asceric Fan of Knives Master’s Training Dummy 2 1 (O: 1470)
01:13.946 Asceric Fan of Knives Master’s Training Dummy 3 1 (O: 1393)
01:13.946 Asceric Fan of Knives Master’s Training Dummy 4 1 (O: 1282)
01:13.956 Asceric gains 2 Energy from Asceric’s Focused Attacks
01:13.956 Asceric gains 2 Energy from Asceric’s Focused Attacks
01:13.956 Asceric gains 2 Energy from Asceric’s Focused Attacks
01:13.956 Asceric gains 2 Energy from Asceric’s Focused Attacks
01:15.857 Asceric casts Fan of Knives
01:15.857 Asceric gains Berserk from Asceric
01:15.860 Asceric Fan of Knives Master’s Training Dummy 1 1 (O: 715)
01:15.860 Asceric Fan of Knives Master’s Training Dummy 2 1 (O: 732)
01:15.860 Asceric Fan of Knives Master’s Training Dummy 3 1 (O: 707)
01:15.860 Asceric Fan of Knives Master’s Training Dummy 4 1 (O: 721)
01:15.860 Asceric Fan of Knives Master’s Training Dummy 1 1 (O: 1404)
01:15.860 Asceric Fan of Knives Master’s Training Dummy 2 1 (O: 1420)
01:15.860 Asceric Fan of Knives Master’s Training Dummy 3 1 (O: 1437)
01:15.860 Asceric Fan of Knives Master’s Training Dummy 4 1 (O: 1348)
01:15.860 Asceric gains 2 Energy from Asceric’s Focused Attacks
01:15.860 Master’s Training Dummy 2’s Blade Twisting is refreshed by Asceric
01:15.860 Asceric gains 2 Energy from Asceric’s Focused Attacks
01:15.860 Master’s Training Dummy 3 is afflicted by Blade Twisting from Asceric
01:15.860 Asceric gains 2 Energy from Asceric’s Focused Attacks
01:15.860 Asceric gains 2 Energy from Asceric’s Focused Attacks
01:15.860 Asceric’s Greatness fades from Asceric
01:16.945 Asceric’s Felstriker fades from Asceric
You can see I get a felstriker proc (which is in the offhand) and it immediately generates 8 energy from the 4 MH crits (much higher damage due to a much higher level weapon being used there). The very next time I cast Fan of Knives, you can see that all 8 attacks clearly hit and are crits, but once again only 8 energy is generated.
On this next log, I equip a Sharp Dagger in my OH (1-2 damage range dagger), with Dalaran Knuckles int he MH. I also unequip all other armor and trinkets To illustrate this interaction more clearly, and demonstrate in the log that auto attacks ARE proc’ing Focused attacks from the Off-hand, but Fan of Knives hit from the OH are not.
Once again, the relevant section is here.
00:00.000 Asceric Fan of Knives Master’s Training Dummy 1 1 (O: 70)
00:00.000 Asceric Fan of Knives Master’s Training Dummy 2 1 (O: 35)
00:00.000 Asceric Fan of Knives Master’s Training Dummy 3 1 (O: 34)
00:00.000 Asceric Fan of Knives Master’s Training Dummy 4 1 (O: 71)
00:00.000 Asceric Fan of Knives Master’s Training Dummy 1 1 (O: 65)
00:00.000 Asceric Fan of Knives Master’s Training Dummy 2 1 (O: 68)
00:00.000 Asceric Fan of Knives Master’s Training Dummy 3 1 (O: 53)
00:00.000 Asceric Fan of Knives Master’s Training Dummy 4 1 (O: 62)
00:09.843 Asceric casts Fan of Knives
00:09.843 Asceric Fan of Knives Master’s Training Dummy 1 1 (O: 34)
00:09.843 Asceric Fan of Knives Master’s Training Dummy 2 1 (O: 35)
00:09.843 Asceric Fan of Knives Master’s Training Dummy 3 1 (O: 35)
00:09.843 Asceric Fan of Knives Master’s Training Dummy 4 1 (O: 35)
00:09.843 Asceric Fan of Knives Master’s Training Dummy 1 1 (O: 55)
00:09.843 Asceric Fan of Knives Master’s Training Dummy 2 1 (O: 49)
00:09.843 Asceric Fan of Knives Master’s Training Dummy 3 1 (O: 60)
00:09.843 Asceric Fan of Knives Master’s Training Dummy 4 1 (O: 64)
00:09.843 Master’s Training Dummy 4 is afflicted by Blade Twisting from Asceric
00:15.220 Asceric casts Fan of Knives
00:15.220 Asceric Fan of Knives Master’s Training Dummy 1 1 (O: 34)
00:15.220 Asceric Fan of Knives Master’s Training Dummy 2 1 (O: 34)
00:15.220 Asceric Fan of Knives Master’s Training Dummy 3 1 (O: 34)
00:15.220 Asceric Fan of Knives Master’s Training Dummy 4 1 (O: 34)
00:15.220 Asceric Fan of Knives Master’s Training Dummy 1 1 (O: 55)
00:15.220 Asceric Fan of Knives Master’s Training Dummy 2 1 (O: 65)
00:15.220 Asceric Fan of Knives Master’s Training Dummy 3 1 (O: 58)
00:15.220 Asceric Fan of Knives Master’s Training Dummy 4 1 (O: 108)
00:15.220 Master’s Training Dummy 2 is afflicted by Blade Twisting from Asceric
00:15.234 Asceric gains 2 Energy from Asceric’s Focused Attacks
00:17.866 Asceric’s Blade Twisting fades from Master’s Training Dummy 4
00:22.914 Asceric casts Fan of Knives
We can clearly see that the off-hand hits all hit the target dummies first (the 34-35 hits, which when crits are 70-71), but the only Focused Attack proc we get is from the very last MH hit on Master’s Training Dummy 4.
Further more, later in the log I spend some time just Auto Attacking a single dummy, and we can see a few different instances where off-hand auto attacks that crit do result in a focused attacks proc.
00:44.098 Asceric swings at Master’s Training Dummy 1
00:44.098 Asceric Melee Master’s Training Dummy 1 1 (O: 27)
00:44.405 Asceric swings at Master’s Training Dummy 1
00:44.405 Asceric Melee Master’s Training Dummy 1 Miss
00:45.553 Asceric swings at Master’s Training Dummy 1
00:45.553 Asceric Melee Master’s Training Dummy 1 1 (O: 27)
00:45.553 Asceric swings at Master’s Training Dummy 1
00:45.553 Asceric Melee Master’s Training Dummy 1 1 (O: 75)
00:46.773 Asceric swings at Master’s Training Dummy 1
00:46.773 Asceric Melee Master’s Training Dummy 1 1 (O: 57)
00:46.973 Asceric swings at Master’s Training Dummy 1
00:46.973 Asceric Melee Master’s Training Dummy 1 1 (O: 27)
00:47.884 Asceric swings at Master’s Training Dummy 1
00:47.884 Asceric Melee Master’s Training Dummy 1 1 (O: 75)
00:48.406 Asceric swings at Master’s Training Dummy 1
00:48.406 Asceric Melee Master’s Training Dummy 1 Miss
00:49.147 Asceric swings at Master’s Training Dummy 1
00:49.147 Asceric Melee Master’s Training Dummy 1 1 (O: 67)
00:49.917 Asceric swings at Master’s Training Dummy 1
00:49.917 Asceric Melee Master’s Training Dummy 1 Miss
00:50.349 Asceric swings at Master’s Training Dummy 1
00:50.349 Asceric Melee Master’s Training Dummy 1 1 (O: 74)
00:51.398 Asceric swings at Master’s Training Dummy 1
00:51.398 Asceric Melee Master’s Training Dummy 1 1 (O: 27)
00:51.528 Asceric swings at Master’s Training Dummy 1
00:51.528 Asceric Melee Master’s Training Dummy 1 1 (O: 128)
00:51.531 Asceric gains 2 Energy from Asceric’s Focused Attacks (O: 2)
00:52.716 Asceric swings at Master’s Training Dummy 1
00:52.716 Asceric Melee Master’s Training Dummy 1 1 (O: 141)
00:52.716 Asceric gains 2 Energy from Asceric’s Focused Attacks (O: 2)
00:52.852 Asceric swings at Master’s Training Dummy 1
00:52.852 Asceric Melee Master’s Training Dummy 1 1 (O: 27)
00:53.907 Asceric swings at Master’s Training Dummy 1
00:53.907 Asceric Melee Master’s Training Dummy 1 1 (O: 123)
00:53.907 Asceric gains 2 Energy from Asceric’s Focused Attacks (O: 2)
00:54.227 Asceric swings at Master’s Training Dummy 1
00:54.227 Asceric Melee Master’s Training Dummy 1 1 (O: 56)
00:54.329 Asceric gains 2 Energy from Asceric’s Focused Attacks (O: 2)
The bolded section above is an offhand crit, that clearly procs Focused Attacks.
So clearly the interaction between Fan of Knives and Focused attacks is bugged. And, to demonstrate this even further, we have video evidence from the original release of “Wrath of the Lich King” that demonstrates Focused Attacks should proc off of crits from both weapons.
The video is titled “Mutilate Rogue Efficiency: Fan of Knives” by the youtuber “Adrew Barrows”. At around 3:15 in the video, the player is engaged with 17 enemies and the video shows clearly a jump of at least +42 energy after a cast of Fan of Knives, which would require 21 triggers of Focused Attacks. This can only happen if Focused Attacks can trigger from both mainhand and offhand hits. If it only triggered from mainhand crits, we would see at most +34 energy and the player would not max out on energy immediately after casting the Fan of Knives.
TL:DR - Fan of Knives is bugged when interacting with Focused Attacks. It was not this way on the original release of Wrath. We have video evidence proving how it interacted back then, and logs currently of how it interacts now.