Update: I tried this battle again, and this time I took Wyll with me to the battle with Kagha. Once it was done, Wyll was walking away from the rest of the party. When I talked to him, the dialogue was like he had never joined the party or the camp in the first place. I picked the option to have him join, he said the party was "full up," even though he was in the party. He kept walking away, while still showing up as a member of the party.