Although libraries support both formats of sign before and after DIGITS(ex: https://github.com/moment/luxon/pull/683, https://github.com/moment/moment/issues/2408), many do not.

For example PG refers to https://www.ietf.org/rfc/rfc3339.txt when converting(Ref: https://www.postgresql.org/docs/current/datatype-datetime.html)

According to the ref there is no explicit mention of allowing sign before the parts, which reads as below:

 Durations:

    dur-second        = 1*DIGIT "S"
    dur-minute        = 1*DIGIT "M" [dur-second]
    dur-hour          = 1*DIGIT "H" [dur-minute]
    dur-time          = "T" (dur-hour / dur-minute / dur-second)
    dur-day           = 1*DIGIT "D"
    dur-week          = 1*DIGIT "W"
    dur-month         = 1*DIGIT "M" [dur-day]
    dur-year          = 1*DIGIT "Y" [dur-month]
    dur-date          = (dur-day / dur-month / dur-year) [dur-time]

    duration          = "P" (dur-date / dur-time / dur-week)

We should not attempt to move sign forward in this case.