I was just doing Twilight Arbor story mode. An hour and a half in, we were at the very end, but one of the group members went offline. We waited for a few minutes, and he didn't come back. So we had the idea to kick him from the group and replace him. Unfortunately, kicking him crashed the instance and kicked everyone out, and reset it when we went back in. Is that the way it's supposed to work, to make it so that party members are irreplaceable? Is that a known bug? Or did my group just get unlucky?