Consider these two sentences:
But aren’t you a superhero?
But aren’t you a superhero.
One of them ends in a question mark while the other ends in a full-stop. My grammar checker doesn’t respond to either of them. So, which one is correct?
Your grammar checker is confused by But.
Use a question mark for a direct question, like your first example. Use a period for an indirect question. Your example can only be a direct question so it takes a question mark.
You are a superhero.
You are a superhero?
There are occasions when either construction is fine depending on what you’re trying to accomplish.
You are a super hero!