It's the normal way to contract 'am not' in Irish-English. It at least has the advantage that it's consistent with the other negative contractions of 'to be' in the present tense.
Whether something is 'proper English' is a slippery subject. Even the most formal and 'correct' English written or spoken today would appear quite the opposite to an English speaker from 200 years ago.
"I amn't" is a standard part of Hiberno-English. AntOlstar's 2 year old comment implies that he entered "I amn't musical" and Duolingo didn't accept that answer, even though that's how many Irish people would answer that question.
It's likely that "I amn't" has been added as an acceptable alternative answer for this question (I think there would be more comments on this thread if it was marked wrong every time a user from Ireland answered "I amn't musical").