Edit (Statement from Double Fine):
Because it helped so much, this issue should be resolved with the next patch we release. You will have to trigger some form of it to get it to go off though (As in for winx7/Corrupted, complete the game with just one of the characters in that way and you'll get it if you are supposed to).
Hey guys! I just want to say first thank you for all the research you guys did into the topic, it helped a TON.
So, I've been at it all day and completed the game with all 7 characters (bad ending), but I did not get the "Corrupted" achievement.
Corruption — Saw the darkness in all our hearts.
To get the bad ending for all 7 characters, for each of them simply hand in someone else’s “Object of Desire” to the clerk to get your own “Object of Desire”, and then climb up the ladder to leave and see the ending.
Update for those who don't want to read the whole thread:
The achievements affected by this bug are just Win-Win-Win-Win-Win-Win-Win, Corruption, Redemption, The Whole Story, and Don't Fill Up on Fortune Cookies, correct? Any others I should be pulling this stunt to make sure I get credit for?
I'm happy to report that yes, saving your game just before finishing the game, and then starting up with keyboard only works for triggering the bugged achievements. (I outright unplugged my gamepad while the game was off, to absolutely sure I'd be on keyboard only mode). Sure enough the Twins went up the ladder, triggering Septuple Win, and then the Adventurer went up, triggering Corruption, exactly as they were supposed to. For the record, I saved before actually acquiring any of the objects of desire.