Anyone knows how to fix the issue of maps resetting? I'm currently stuck in the first (sneaky) visit to Castle Ravenloft (not the invite one) and cannot exit as the map reset and the door is locked.

I'm confused how some of you manage to get into this situation. Anyway, see this thread:
https://www.gog.com/forum/dungeons_dragons_ravenloft_series/stuck_inside_castle_ravenloft_no_exit

Did you install the game into some protected Windows folder? It seems the game isn't allowed to store files in its work folder as you're playing.

I first installed the game in the default GOG Galaxy folder. I then uninstalled the game, changed the download/installation folder to another folder outside GOG Galaxy and redownloaded the game and it has the same issue. Not sure what else I can do but it makes the game unplayable.

I'd try creating a support ticket directly to GOG, since they don't really read these forums.