Ok this is not really specific to Cult of the Lamb, I see this issue on a lot of games.

Example : I couldn't unlock my last skin in the game because of no snails appearing in dungeons.
The game was updated, I launched it, which made Galaxy crash, but the game runs fine. (this galaxy crashing at game starts happens to me quite often for months with any game)
I forgot that galaxy is crashed and play a bit, finally find snails, unlock the skin, don't see any achievement notification, remember that galaxy was crashed, close the gamen restart galaxy, restart the game, load my save, even pick a new follower and choose the snail skin, but no update of the achievement.

I can't say if that's more Galaxy/GoG or the game at fault, and I'm not very much attached to achievements (even if I would have liked to 100% this one), but I wanted to notify this.
I would say that it could be unlocked through another game-slot, but I can't be 100% sure.

Ok for the record : the achievement triggered as soon as I entered a dungeon.