PSA: Don't use the words 'red flag' to describe just a thing you don't like. Not even as a joke.
Red flags are warning signs that someone is dangerous or toxic. Things like "snaps at people working in food service," "brushes you off when you try to talk about something that bothered you," or "doesn't believe in therapy." It doesn't have to always mean romantic relationships, but that's where it's often used.
Someone who gets the mushroom cave in Stardew Valley instead of the bat cave is not dangerous or toxic. (Yes, I have seen that.)
Someone who's into (SAFE, SANE, AND CONSENSUAL) kinks you're not is not dangerous or toxic.
Someone who eats food a way you don't is not dangerous or toxic.
You can just dislike something without it having to be a moral issue. Crying wolf where there's no danger just because you don't like something not only insults the other person, but diminishes the ACTUAL meaning of the word when the whole reason we call out red flags is to help people realize they might be in danger.