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If I remember correctly, you'll need to get Affinity with Horace. I don't remember which, but you'll get a cutscene with Arthur and Horace, trying to get a key from him. After that, you'll be able to purchase the underwear from the Bandit! Hope that helped! :3

So far, it seems to have been happening in the morning time

Found a bug specifically with battles in Feasterra, where when you attempt the fight for vegetables, you get a blackscreen