Well this is embarrassing, and exactly why I wanted to avoid contacting you directly until somebody(GOG or one of the forum users) could confirm or deny if the issue was wide spread. As it turned out, it was an issue just on my PC.
Once I received your indication that the game is working correctly on your end, I searched around and while my antivirus wasn't interfering with the game, my firewall's HIPS module did have a rule blocking some action the game was attempting. Apparently not enough to crash the game, but enough to interfere with Galaxy detecting the achievements. As soon as I removed that rule, Galaxy correctly detected the next achievement I unlocked in the game.
This has never happened to me before, so I didn't even suspect it might be something possible. Lesson learned.
I apologize for wasting your time, geb88, I had you searching for ghosts. Thank you very much for your response and help, and good luck developing the game further, or working on future "books" in the series. ;)
As you can see Alexim, it was entirely an issue on my end. Everything is now working correctly. Please remove Book of Demons from the list.