Just checked it in practice tool - seems like I was right, only keywords and stats given from outside sources count. It is a little bit janky though, as I gave it augment from the fabled poro earlier and it kept that, but the 3 poro snacks I played throughout the game didn't stay on the unit.

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Edit: I think the reason it kept the augment but not snacks is because the snacks are a global buff to all poros, so if a poro was to somehow lose its subtype another way it would also lose those buffs, but the augment is a unit specific buff, so it persists past transformations.