Hey! Long time no see. Turns out Merry Bad End has been broken for around two years? I suspect the issue has to do with GameMaker changing how it handles Steam's API, although that shouldn't have retroactively broken it ... but I digress. You now only need one bad END to trigger the achievement, so you patient achievement-hunters can finally get that 100%.
I'd like to fix it properly, but I think I'm going to need to update the whole game to the latest GameMaker edition, and that's going to be a Whole Thing and there are some other fixes I'd like to make if I'm doing that, so for now this'll have to do.
The way the code works, you'll need a save that has at least one bad END as-yet unachieved. (If you don't want to start fresh, you can email me at [email protected], I have one handy.) As soon as you achieve the END and return to the void, the achievement...won't pop, for some reason, but will show up on your account. Hopefully. If it doesn't, that's a problem, so let me know.
Thanks again to everyone who both brought this to my attention and very, very patiently waited for me to return.