
If it only touches the hair, then not offside, from FIFA rules
The hair by itself is not considered a body part upon contact with the ball, so if the ball only touches the hair without any influence on the ball, it's not considered contact. That would mean no contact was made because the ball was simply "missed" by the head.
The referee/VAR was not prepared for the event because it's so rare. Hair alone would likely be enough to show the ball was touched by their sensors, but not illegal contact with the ball that would make it raise the offside rule.
Or we might be wrong about this interpretation. Right now I think it was a robbery. But maybe they'll have more data for everyone tomorrow that can show it touched more then his hair....