Not my ISP (Init7 FTW!), but my router (Mikrotik) is notoriously infamous for being a total crap at IPv6 (see for example https://michael.stapelberg.ch/posts/2021-05-28-configured-an...)

In my particular case there seems to be an odd bug / misconfiguration from my side that makes the router / clients from time to time loose the IPv6 routing. The fallback is... a connection hanging forever. The only fix? Reconnecting to the Wi-Fi to get refresh the DHCP lease.

I debugged it for waay too long, and at this point I'm 80% convinced it's a Mikrotik bug of some sort.