It sounds like you may have accidentally ended Gale's turn by mistake.
I've never had any issues with NPC's overtaking turns, nor any issues with interacting with not in combat party members while one engaged character is stood around waiting for a turn (Or even using said party members while NPC's are taking turns)
As such it sounds very much like what would happen if you passed Gale's turn. Which you may have done without realizing.
That said, if I recall correctly, that particular encounter at Moonrise does have a unique interaction whereby the Harpers decide to engage on their own if you wait long enough. So there is a possibility that this behaviour caused this issue, whereby their auto-engage activated when you selected another party member that wasn't in combat.
I’m playing unmodded, as the Honour-Mode achievements wouldn’t work otherwise.
Fun Fact: Mods don't necessarily affect Honour Mode achievements. Many mods (Including save files made right at the end of the game, mods that add in god items that make you invincible and ultra-powerful and even mods that adjust difficulties and balance mechanics like removing Honour Mode specific features) will still allow achievements to be obtained.
