Yes, it's a bug, and a long standing one at that. Mission vox allows players from other teams to hear your voice transmissions...don't use it. Instead, use a numbered voice channel between 1 and 200, (over 200, other team players can get on it.)
Imo, what probably happened is Ute may have overheard your mission plans vis the mission channel, completely by accident, and, bad luck for you.
As far as him showing on the mission roster as well, that's something I haven't seen, but it could be related to the mission voice bug somehow.
Did you ask Ute about it. What did he say?
Btw, I don't blame you for being pissed. Never use mission voice, it doesn't work right.
Les