I’ve experienced the same bug as you starting about over a week ago, matching the date of the latest patch.
My problem is the same, that suddenly the combat music won’t start during an encounter, and instead the exploration music keeps playing. I haven’t found a solution yet either, though what I have found is; Normal music change, such as when you speak to certain NPC’s that have a different background music and the like works perfectly fine at the correct times, but Combat music just fails to start in general, with one exception; Whilst in combat, if a large enough group of enemies or summons appear at the same time during turnbased combat (Haven’t tested without turnbased) The Combat music will kick in, replacing the exploration music. However, when the encounter ends, the combat music will keep playing, until you either reload a save, or you enter a dialogue with an npc with a specific background music, though the combat music starts right ahead after the dialogue is finished.
That’s all I’ve figured out so far, sent a few messages to Owlcat games too but got no replies yet.
If anyone else get the same problem, do tell, especially if you find a solution as well