This must be one in a million. I was battling some mobs in the Ancient Temple and when I defeated them, the battle did not end. A "Countess Mal Bloodthorne does not have a valid target" error message popped up on the last turn (the turn I defeated the last enemy). Unfortunately I did not screenshot that, and the battle log coincidentally reset.
Now I'm stuck in the battle and can't escape it.
Edit: I was able to escape the battle.

In the future, if you (or anyone else) runs into this same issue, just skip everyone's turn and then the battle should process after that turn.

Logically, this scenario should be impossible. So obviously I don't know what causes it lol. I've looked before, but can't find anything that jumps out at me. Without knowing exactly how to reproduce this on demand, it will be virtually impossible to ever fix.