>>29871774To be clear, it works on "Gift" Pokemon, not "Event" Pokemon.
A "Gift" Pokemon are the ones you receive in-game from an NPC, like Castform in RSE or Type:Null in SM. Not only does it work, but it works 100% guaranteed. The Received Pokemon will always match the Synchronized Nature.
An "Event" Pokemon is anything you receive that's associated with Mystery Events, so the 20th Anniversary Pokemon wouldn't work.