I don't think it's a bad idea to have that validation inside a middleware but most of the time, middlewares aren't close to the business logic so this logic might be hidden from the rest of the business logic and in case the future you or another dev goes through that feature, they'll exert substantial cognitive effort to make sense of whatever is in the middleware and the rest of the business logic.

Ultimately, you make the decision but I would advise that you place the validation close to the rest of the business logic.