Party members stuck in delve

Bug report # 1,126,237,852
https://steamcommunity.com/sharedfiles/filedetails/?id=3685945203

The ability to interact with the Subterranean menu is not appearing for me in a buddy's delve. I can't go up to the Mine Encampment, and I can't portal out.

Items are on, tried hitting Z a few times just to rule that out, threw stuff on the ground to verify.

EDIT: Additional info, when I use his yellow portals I disconnect. I can only join him with the blue swirly method.

EDIT #2: He gets the same symptoms in my delve.

I have the exact same behavior - can't leave the delve as a party member in someone else's delve. I haven't encountered the crashes yet but the missing UI in delve is annoying - I can't go refill flares without logging out and logging in again.