The problem is, it is hard to define what a "parent" or "core" elf race is.
Some elves have darkvision, some elves have sensitive darkvision, and hopefully, some elves can lack it and have standard day vision.
It is more helpful if each race concept stands on its own, fully expressed as a comprehensive character concept, without getting compromised by an obligation to conform to a parent or core.
Same goes for the gith, and so on. If two concepts within the same race are meaningfully different, let them be different.