I've noticed this while playing a hypocritical Killer Vegan/Necronomiconomicsbuilds. If I use a cursing ability first (doesn't matter what the target is, it works even if you strike the same animal you just attack) and next action strike an animal I don't get hit with the Fallen Vegan debuff. I'd guess the Necronomiconomics debuffs override the Killer Vegan ones. Eating a forbidden food afterwards however is still very much not okay. It lets you get away with animal abuse but not eating meat or cheese. I have all expansion packs.
I thought this was an intended feature (because they would be hostile towards you) but I dont have much experience in the Vegan skill tree
Dredmorpedia says the melee debuffs only have a 50% chance to occur (my Vegan only ever attacked once and got it); how many times have you tried this? Maybe you were just lucky. If it happens 20 times in a row then it's probably a bug.
It happened quite a few times, I've only had melee-hit after swing ones happen I think two or three times. In those cases I suffered Fallen Vegan and then got cursed next turn. I've never seen it happen on the same turn I suffered a curse.
