TL;DR
When you craft a shield you have a 1/8 chance of getting a UV. That UV can be damage type (Norm/Ele/Pierce/Shadow) or status (Stun, Fire, Shock, Poison, Curse, Freeze).
When you Punch a shield you can only get damage type UVs. You cannot get status UVs at Punch.
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I've been asking around--nobody has a good explanation as to why you can't Punch a status UV (Stun, Fire, Shock, etc) onto a shield.