Last I read this is how Instagram rate limits IPV6. See https://news.ycombinator.com/item?id=32687694

So apparently not enough false positives for them to care.

Is it common for mobile networks to hand out short (64 or smaller) prefixes to phones? Phones are routers after all (hotspot) so this would seem to break stuff.

people don't care about false positives on CGNAT either, so not much difference to the IPv4 situation if you target /64s