The とはかぎらない grammar point is only accepting とはかぎりません when asked for the polite form. とはかぎらないです is counted as wrong. Unless there’s something special about this grammar point, it should be counted as correct (or at the very least yellowed out to try again) like every other polite verb conjugation, right?
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Hi there,
Nice catch! This should now be fixed.
Have a great day, and happy studying. 
Sean
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