Currently (Bad) Endings are just the defeat screen you get when losing with a lot of body parts for a certain tf. Basically you get a variation of the ending screen depending on the trasnformation you had. Nothing special yet. "Actual" endings are currently not implemented since the game cannot be beaten yet. But I am planning to have different endings depending on transformations in the full game.
Oh almost forgot they also only appear when losing because of corruption if I remember correctly. But since you haven't seen those you didn't get the Alraune Bad end which woud have triggered the achievment so it might be a bug.
I will try to replicate it when i have time and get to the bottom of this.
Thanks a lot for the reports!