that's probably because, apart from on these boards, not many people are actually that aware that in the Welsh language -wyn is a male ending. And those that do know, probably don't care. Even in Wales itself, you'll find the odd female
Bronwyn, and probably no one ever bats an eyelid (
My cousin's Welsh boyfriend, who moved to England four years ago, claims to have had no idea that
Bronwyn is technically 'incorrect').
Besides that, if the name is Maori in origin, then there's no reason why anyone SHOULD use -wen rather than -wyn, whether it's for a male or a female.