I've gotten quite a few of these bugs too. On my last Act 1 playthrough, most of them turned up this way, despite me completing all the quests.
The seeker entry even said I "didn't use the Seeker's help to get off the island", even though I had them move to the forward camp, and I cleared the shriekers. I walked too far towards Alexander before I talked to Gareth one last time, so they all moved to the boat.
