Just to add to it.
I tried to create a party in the method described, by hiring a seventh adventurer. I made a rogue, dismissed BB Rogue, left and when I went back into the inn, it brought up the party management UI without my requesting it. BB Rogue was waiting in the wings, and everything looked fine.
I saved and loaded after that, while still in the inn. When I loaded, my new rogue was in the inn but not in the party, and BB Rogue was back in the party. When I went to manage the party (by pressing "P"), I saw a white box in the "waiting" section that when I hovered over it, said "BB Rogue", who was still in my party... which was weird. All I did with this screen was look at it, I didn't click on anything since this is known to result in transition problems.
So, I went and hired a seventh adventurer again. I was unable to remove BB Rogue from the party, and when I clicked on the white box (even while already having six party members), I was able to recruit a duplicate of BB Rogue. I now had two BB Rogues and not my new rogue, and when I left the tavern I encountered the transition issue.