Yeah, I just seem to have stumbled over a weird little bug in Blackwell Convergence (forgot to put that in the title, can't edit that - sorry!)

I played the game for a while, noting how Joey says "we could have handled" that a bit better right at the end of getting the first ghost into the light, and how I had recieved the achievement for staying and drinking at the party. Then I noticed that one achievement seemed to be connected to Joeys remark and how there was another that seemed to be connected to NOT staying and drinking. Well, I figured I'd start a second playthrough (because I am obsessed :P), and quickly got those two achievements. I made a savegame and continued with my original playthrough. At that point all achievements were still in the list.
Here's the bug: I later overwrote the save with a new one, figuring I didn't need it anymore.
And the two achievements from are gone.