Ok, thanks, guess I’ll stick with Ubuntu 20.04 for as long as possible, hopefully IPv6 support improves until then. I’m really surprised by the amount of IPv6 bugs in Debian, some of them even over a decade old, for example simply not being able to configure sssd without IPv4.

Yeah, SLAAC works as expected on Debian 10-12 LXCs as well, only DHCPv6 has issues.