I agree that this is not a face mask foul. But I don’t think we can judge whether it’s no foul at all. If a player intentionally removes an opponent’s helmet, especially in a violent manner, this could absolutely qualify as UNR or even UNS. Now if it’s just in the process of a normal tackle and that’s what he happened to grab and the helmet comes off, I’ve got nothing. But if he intentionally grabs the helmet and pulls it off on purpose, I would support a foul.