
Anyways, in Bordermarch you get a quest from Grum Balder to hunt down some Bugbears. You enter the cave and watch a cutscene, and all is well. BUT the second you enter the main cave all of your companions will be despawned/removed from the game.
So any summons/mercenaries/whatever will just disappear. It doesn't say anything in the log about them unsummoning or despawning. They just disappear.