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!
My approach is when you have some sentence with you containing "but", you need to consider the remaining part of the sentence ignoring the but and also check the punctuation.
Consider the following two sentences: 1. But, it is OK. 2. But, aren't you a super hero?
Removing but they still carry their sense. 1. It is OK. Consoling someone. Ended with period. 2. Aren't you a super hero? Questioned someone if he is a super hero or not.
It ultimately requires question mark.