Is it unusual to be asked by Asia miles when signing up, to forward a copy of your passport?
It’s a tiny bit unusual to be asked for a passport copy, rather than simply your passport number. It’s not unprecedented, though. I have a vague recollection of needing to send a scan of my passport to Asiana when I joined their FF programme. Certainly Cathay Pacific/Asia Miles has a copy of my passport number.
I wouldn’t worry about it. Take all the usual precautions against phishing and hacking (eg: log on to the Asia Miles website directly yourself rather than clicking on a link, don’t use an unsecured wi-fi network, etc), but the bottom line is that Asia Miles is a reputable FF programme and they can probably be trusted as much as any Chinese company or organisation to look after your data.