A character is hit by stun or feeblemind and after the battle you realize the character is gone from your party list. He's still on the screen affected by spell, but has a blue circle underneath him and you can't initiate conversation.
This has happened twice, once to my fighter and once to my thief. I didn't accidentally drop the character either. This happens exclusively during battle.
I've tried to wait it out, and see if I can talk to the character when the spell wears off him, but it lasted longer than an hour of real time. The only way around this has been to reload.
This has only started since I've entered the sewers beneath Temple District and more precisely, since I've encountered the beholders there.

I'm sure you've already checked for this, but just in case you didn't - beholders have a nasty habit of tossing petrification rays. I can't remember if that will give the blue circle under the charater, but it will drop them off the list and cause them not to respond indefinitely. If it indeed is a hold or feeblemind spell that's causing the problem, I'd say you have a definite bug.

I'd go with petrification, too. That's exactly what it does. Out of the party, blue circle. You can use stone to flesh on them to fix it up.