I was leveling Bastian. He was in his standard, white gear except blue lightning staff I found. I got invited to do random normal that I accepted. We did the dungeon (it was Fang Lair), then one person left. My Bastian automatically appeared, but he was naked, I checked companion menu and all the gear is gone, I dont have it anywhere in my inventory.
The group join was in Craglorn if that matters
So i guess, despawn your companions before queing.