I just did a Bounty wherein I killed a Malevolent Tormentor and an Odious Collector. Neither the Bounty Cache nor either goblin dropped a plan.
I have all other level 70 versions of the low-level plans, but no Captain Crimson.
Blizzard, I think it’s time for you to seed this plan into the game somewhere other than as a drop from where it is supposed to drop. Because as you can see, it isn’t.