I have one achievement left and it's hidden one. I have no idea what I have to do as I've done pretty much everything there is to do in the game.

From an online search I found that it might be the 'Director of the FBC' achievement that I should have gotten once I got all the other achievements. Is that the case? If so, is it just a bug?

Thanks.

It's probably the director cheev, and yes, achievements for Control have been more or less broken since launch on GOG. Achievements in general on Galaxy are more or less broken.

It shouldn't be Director of the FBC because Director of the FBC isn't a hidden achievement (it also has an earn rate of 0.1% because it's broken, so it's not that one either). Unfortunately I also don't have this achievement and can't tell you what it is.

I've been talking to people about this in the broken achievement thread in General discussion and it seems my seeing the Director of the FBC is a Galaxy error of some sort, so my bad. I'm afraid I don't know anything about the last hidden achievement.