Something to do with Router Advertisement intervals being too short, though I don't get why that only affects her ~5yo android phone. And IPv6 is so complex, I haven't figured out if the RA interval is something I can or should tweak, whether that comes from the PiHole or whether I'd have to flash OpenWRT on my router, or whether my ISP ultimately controls that upstream. Like, I can't figure out as easily where the boundary between me and "the internet" ends with things like the /64 prefixes and SLAAC and RDNSS and all the other acronyms.
Yeah, yeah, I should RTFM, and eventually I might figure out what makes a "good" home IPv6 network. But I can't be arsed to do that in my free time yet, and neither can most software companies cough cough Google/Android and that one guy causing IPv6 drama in the android team
Like.... Ehhh... I'll come back to it in a few more years. "Are we IPv6 yet?"