I also experienced this. Just started playing the game for the first time and had recruited two adventurers. The bug occurred after Serafen joined my full party and I switched out an adventurer to make room for him. That particular adventurer is the only one bugged. I still see him as a choice when managing the party, but when I click OK he is not in the party.
Have tried:
* to remove all other chars from party
* quit, load, save
* add a new adventurer to party (this worked fine - all other characters can be switched in and out of party except my favorite adventurer)
* validating game files in steam
Also of minor note: looking at the savegame pictures, Aloth appears twice, both in the slot he is in but also in the slot my adventurer was in.
I suppose I could just clone him in the tavern and suck up the money and xp loss, but he is also had a nice sword found in the digsite