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In the bulleted list of individual regular expressions for durationLexicalRep (https://www.w3.org/TR/xmlschema11-2/#nt-duTFrag), the first regular expression ("only strings in which the fields occur in the proper order") is: -?P[0-9]+Y?([0-9]+M)?([0-9]+D)?(T([0-9]+H)?([0-9]+M)?([0-9]+(\.[0-9]+)?S)?)? Parentheses are missing around the year number. It should be: -?P([0-9]+Y)?([0-9]+M)?([0-9]+D)?(T([0-9]+H)?([0-9]+M)?([0-9]+(\.[0-9]+)?S)?)? The all-in-one regular expression appears to be correct, but it does contain superfluous parentheses around required fragments.
(Personal response). I think you are right; thanks for reporting it. There's currently no active maintenance project for the XML Schema specifications but the bug will presumably be picked up if work on XSD is resumed in the future.