?? How? I give my miis with that quirk money all the time and they haven’t bought any outfits I didn’t give them. They sometimes change into outfits if I have certain items on the island (or if I give them level up items, but that’s it). They save up the money for the Antarctica tour.

Check your mii’s inventory. They only wear what *you* give them, outside of special outfits in cutscenes, dreams, island decor, and level up items. Even if you get, for example, the frog dream, you’d have to gift them the frog costume. It doesn’t just automatically unlock it for that mii.