Okay, so I figured it out for my specific case with Minthara. I'm playing Durge and have the Deathstalker Mantle, which turns you invisible for 2 turns after killing an enemy. Seems like when you are invisible Minthara can't see you, so she just picks a different party member instead. In my case it was always Astarion. No clue why him, he wasn't the closest to her (literally on the other end of the room) nor is he next in the party list (on the right screen side).
Unequipping the Mantle solved it for that case.
Last edited by Stinker666; 14/11/23 01:58 PM.