Critical Hits don't work correctly with stats changes.
The situation is a little bit weird but it's the only easy way I found to re-create the bug.
Pokemon used:
- Gliscor with Curse and Baton Pass.
- Reckless Staraptor with Focus Energy and Brave Bird.
- Unaware Quagsire which is holding a Choice Band and has Trick as one of its moves. (Impish max HP max Defence)
Gliscor vs Staraptor.
Gliscor uses Curse three times while Staraptor keeps going for Focus Energy increasing its crit canches.
Once reached +3 Gliscor uses Baton Pass and passes all the stats changes to an Unaware Quagsire.
Staraptor keeps going for Focus Energy.
Quagsire then uses tricks and gives the Staraptor a Choice Band.
Staraptor goes for Brave Bird and can't kill the Quagsire even with a crit.
Quagsire is supposed to be OHKOed by a crit Brave Bird.
As I said, the situation is a little bit weird so I'm going to give you a showdown link of the situation so you can test it better in Pixelmon.
http://replay.pokemonshowdown.com/customgame-535952788Me and my friend are also talking in the chat so you can get a better idea of the situation.
I can also suggest you to try and go to +6 with the Curses and then use Baton Pass and have a look on how different are the damages.
I'm here if more informations are needed.