Um... not exclusively: "Am not... used in questions." https://dictionary.cambridge.org/dictionary/english/aren-t
Not to disrupt your native dispute. We seem to have been accepting "Am I not", "I am not", and "Aren't I" for years. The complicating factor is that some time ago Duolingo elected to show only the main translation instead of the "nearest" accepted answer when correcting the user. Jumping to conclusions is rampant, and whatever that decision was supposed to fix has been paid many times over by explaining non-existent issues time and time again.
For extra fun, Duolingo's automatic contraction handler apparently does not automatically handle accepting "aren't I" for "am I not", but if we add "aren't I" to our accepted list, it is then apparently happy to accept "are not I".
So keep at it, but this problem is right up there with the intractable apostrophe, and we the volunteers are doomed to failure no matter which way.