Right, so, I installed this on launch day and finished it twice offline before I decided to come on Galaxy and have a few games of Co-op. However, when I launched the game through Galaxy for the first time it reset all the achievements I had earned offline. Is this just what Galaxy does or is this a serious bug/glitch on GOG's end? I tried using an older manual backup save I had and ran the game without Galaxy once again but all my old achievements were still gone. Is there a fix for this or do I just have to re-earn them again?

Well I assume that Galaxy "creates a new profile" that you use for both offline and online gaming which is the reason for achievs to get reset. Apart from that dont have any other idea why it reset your achievs.

So, does that means when I play the game offline all my offline achievements should be back to the way they were before I played the game through Galaxy then? because they aren't.