Came across this issue last night. My campaign normally has 4 players, but one was unavailable last night. The three who could play loaded up but were assigned the wrong characters. I attempted to move them around in the Party Management screen, but since the 4th player wasn't playing I was not allowed to reassign character to players. I would switch them around and click accept, but a message saying, "One or more players are unassigned.) or something along those lines.
I had a random 4th person join up briefly and I was able to reassign characters, and then I just removed the 4th player from the game. Is there any other way to do this if I don't have a 4th player available? Is this is a bug or was it designed this way?