I got the Bounty Hunter (monster extermination) a bit early too.
Some of the other achievements seem to be broken, and based on the steam reports, which achievements are broken seem to depend on which version of the game you're running.
I couldn't get Blue Knight from the initial version from release date, but I did manage to get it after the 11.26.2015 batch, but I can't get Breakfast Safari, even though I found them all in a single playthrough as well.
I'm also missing a few others, but since GOG Galaxy doesn't have counters, right now (still going through NG+ 100% playthrough), I can't tell if I'm missing something or whether it's simply not registering.
They are supposed to register right away.