As far as I am aware I do not think that Bounce Back in it’s current form is bugged it’s just a case of awkward wording on Blizzard’s part that makes the talent significantly worse than it should be. Bounce Back states that:
When a hit deals more than 12% of your maximum health, reduce all damage you take by x% for 6 sec.
Hit is the keyword here that really makes the talent quite poor for raiding. A hit is determined by the spell itself and often times isn’t very clearly defined. There can be two mechanics that function identical to each other yet one will proc Bounce Back and the other will not.
From the testing we’ve seen on PTR so far and what we know on live servers it is very hard to determine what will and will not proc this ability. We essentially have to manually go through each mechanic and list off what procs it (very few mechanics).
Hopefully we can get some clarification on how the talent should interact with damage taken because if Blizzard wishes to continually buff a talent for Mistweaver that only procs on 1 or 2 encounters in the instance then it is not very useful.