This happened in an online co-op game. My buddy went to use the respec mirror in the Lady Vengeance during Act 2. We either lost connection right before he entered the mirror or while he was in the middle of it. Upon reloading the game, his character had the stats of a level 1 character, including attributes, combat abilities, and civil abilities. His character information still read that he was level 10.
Unfortunately, I had played a single player file a lot the night before. All the quicksaves and autosaves from our last co-op session were overwritten. I also saved the game after losing connection out of habit of frequently saving. We tried reloading the game multiple times, I tried playing the save file without him in the game, and neither worked. We played a bit further to level up to see if that would reset the character at the proper level, but it did not.
I'm currently in the process of reinstalling the game in the hopes that it randomly fixes whatever went wrong. Is this a known bug, and is there a fix to this situation? We are playing on Windows 10 through Steam.