I've just finished the quest to save Halsin, the journal says to meet him in the Grove. I also completed the Investigate Kagha quest, leading her to turn on the Shadow Druids and await judgement.
When I go back to the Grove, Halsin repeats one line "You endangered the Grove, how dare you show your face" to Kagha in front of him. Kagha just stands there. Speaking to Halsin yields generic "Not now/Later/In a moment" voiceless dialogue, and Kagha repeats a generic "Silvanus protect you" dialogue if spoken to.
I've tried saving/loading, fast travelling away and back, entering the cave and coming back out, switching party, attacking them, and skipping the dialogue by triggering the Tiefling Party. At the Tiefling party, Halsin still just repeats his "not now" line if spoken to, but the rest of the party proceeds as normal. After the party he acts normally, the conversation about Moonrise occurs correctly and everything seems fine. Nothing else had any effect.
It seems this bug isn't game breaking, but it does prevent seeing the confrontation between Kagha and Halsin if she decides to surrender to him.