The theme is glitched. At times when it starts playing and when it finishes, it literally "kills" all the music in the game.

First example: When you finish the said DLC, the music in credits is not playing, which is ironically Hearths of Stone theme.

Second example: When you finish a certain Blood and Wine quest, the same theme plays in the background, and after the quest in checked out as completed, there's no music, not even when playing Gwent.

The fix is to restart the game, which is stupid.

Note: I did not experience this glitch during my first playthrough.