Nationality issues are rarely fair, if you move to another country, links to the old one can and often will disappear.
I imagine she has Right to Land then, which is still better than nothing. After the handover, if people from HK immigrated abroad and had children, most likely they wouldn't be eligible for anything.
Technically speaking not all HKIDs can be temporary, RTL ID cards are the non-permanent type, but they are effectively permanent. But I guess for most people on other visas, the ID depends on the visa, so I guess in most cases it's temporary.