A DESCRIPTION OF THE PROBLEM :
The negative amount examples sited are not valid, and do not parse.

    "P-6H3M" -- parses as "-6 hours and +3 minutes"
    "-P6H3M" -- parses as "-6 hours and -3 minutes"
    "-P-6H+3M" -- parses as "+6 hours and -3 minutes"

They're missing the 'T' part. And should be:

    "PT-6H3M" -- parses as "-6 hours and +3 minutes"
    "-PT6H3M" -- parses as "-6 hours and -3 minutes"
    "-PT-6H+3M" -- parses as "+6 hours and -3 minutes"

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -

    "PT-6H3M" -- parses as "-6 hours and +3 minutes"
    "-PT6H3M" -- parses as "-6 hours and -3 minutes"
    "-PT-6H+3M" -- parses as "+6 hours and -3 minutes"
ACTUAL -

    "P-6H3M" -- parses as "-6 hours and +3 minutes"
    "-P6H3M" -- parses as "-6 hours and -3 minutes"
    "-P-6H+3M" -- parses as "+6 hours and -3 minutes"

URL OF FAULTY DOCUMENTATION :
https://docs.oracle.com/javase/8/docs/api/java/time/Duration.html#parse-java.lang.CharSequence-