Bleeding out cogs are always ignored by the Locust (unless you are playing Insane mode I believe).
How many COG's are you playing? If two are bleeding out the Wretches/Drones/Boomers will advance towards/attack the ones that are still standing. If you are only playing two COG's (that are both bleeding out)the game is over.
Point is that both are not actually bleeding out, one is only considered bleeding out. COG A plays Cover Fire, which means for the Locust Activation Step, COG B is considered bleeding out (ignored by Locusts), then COG A gets hit and becomes actually bleeding out. Both COG are now, from the AI perspective bleeding out, but COG B is not bleeding out as far as losing the game is concerned.