Larian Studios Forums Baldur's Gate III Baldur's Gate III - Story & Character Discussion Strange reaction from Wyll after skirmish in the druid grove.

You don't mention whether or not you killed the 4 goblins in the cave system under the Grove and freed the Druid they are holding captive. Probably they are the ones in the grove later if you didn't kill them. Just a passing thought as you may well have done and just not mentioned it.
I think the main issue is that somehow you are triggering Halsin and Zevlors thanks and the Tiefling party before dealing with ALL of the goblin leaders (one way or another). That's actually not supposed to happen and is clearly a bug. The Tieflings can't leave if the Goblin armies are intact and with even one leader still being alive they would be (even if you don't see them) and Zevlors quest definitely says kill the leaders [plural] , so there shouldn't be thanks and a party.