RenKalan: It's not a bug. Some soundtracks sold in gog can be bought without having to own the base game. As such, there is a separate entry in the library for the game and the soundtrack.
Tarhiel: Eh... why can´t it be inside one and the same folder, when it belongs together?
Is having a game files (i.e. game) prerequisite for their system to put files together?
The base game is a prerequisite for the majority of dlc's, yes. Then there's no additional entry in your library, dlc's go in the same folder under extras or additional content or whatever.
Some developers though, offer their dlc's (the soundtrack in this case) as a separate product not as an additional content.
Personally, I don't mind but I wish gog sold more soundtracks separately from their games. Some games I don't like but the ost is great and I would like to have the option to buy it without the game.