I have the same thing but with Aloth, and his streaking location of choice is in the great hall of Caed Nua.
I'd had Aloth running some side missions, and decided to just try adding him back to my party. I figured I'd just ignore the fact that he'd lost a bunch of decent items including a soul-bound scepter during his travels (he'd been on a ship that had been wrecked in one of the side quests, so fair's fair), and just be happy that he'd made it back to Caed Nua in one piece. I added him back to the party in the great hall and gave him some new equipment, then left the great hall, walked across the Caed Nua grounds and went into the chapel. Upon leaving the chapel and before being able to tell my party to move anywhere on the Caed Nua grounds map again, the party management screen popped up and I clicked OK to leave the party as-is (i.e. with six members including Aloth), and I got feedback in the speech/activity log that Aloth had left the party. I still had 6 party members on the ground with green circles around them, but Aloth's portrait had disappeared. Holding the mouse cursor over Aloth's person brought up his name in a tooltip above where his portrait should have been. Upon entering the great hall again, I only had 5 members in my party. Adding Aloth straight back to my party while in the great hall revealed that he'd lost all of his newly added and equipped items again. Maybe Aloth's mind has finally cracked due to Iselmyr's presence...
After reloading, I found that if I visit the upper floor of Brighthollow Aloth is standing at the bookshelf with his *ahem* scepter in one hand, and with his secondary weapon hanging from his hip, but with no other equipment visible. I can engage him in conversation, but if I add him to the party via the Stronghold - Party Management screen he does not have any items in his inventory, quick slots or item slots.
Unfortunately my oldest save is from 2 days before, and Aloth "Frank the Tank" was already streaking through the great hall then, as it turns out. It looks like I'll have to start the game again, but I'll be keeping a save game from every time I play from now on.
