I've had it happen on my casters when I'm in a party with someone else's summon. The tool seems to grab the first summoned creature within the party, maybe.

I've done it a few times and the boss has spawned every time.

Had a lot of fun with the quest as a whole, though. Just the right amount of challenge, spaced out well for breathers. Could use some more potions dropping on orc corpses for the sake of replenishment and scrounging mid-combat, but other than what's been discussed I thought I'd just pipe in to say it was fun!