Remember, mirrors in games (if you're not using ray-tracing reflections) are not actually mirrors. They're a character model that must be generated just like your character model in the game. Hence, like any character model, it's possible to assign the wrong assets to it. It could be something as simple as the ID numbers for a body part not matching the ID number used for the "mirror model".
(So, if ObjectID_1234 is my hat on my character, but MirrorObjectID_1234 has a broken texture or something...you don't see a hat in the mirror...you see a bald head. The hair would normally be culled so the hat looks right, but the hat has a corrupted texture, so nothing displays. Or if MirrorObjectID_1234 is mislabled, I might see a red bandana instead of a blue hat. The same thing is true of any body part that can be randomized.)