Leveling up a companion sometimes causes the wrong companion to be leveled.

User Reports

"When leveling up your second slot character, you'll sometimes encounter a bug that levels up another character. It would be fine if the other character shared a class with the character you're trying to level up, but the bug seems to randomly select other members of your party. "

"I found a bug. When I tried to level up my archer, it take to my wizard."