as VM within PVE or physical external ?
is your Edge router have IPv6 ?
IPv6 uses RA to get IPv6 prefix from router then generates its own IPv6 itself , it's not like IPv4 DHCP.
BTW, /etc/sysctl.conf is no more used since Debian13, *.conf are in /etc/sysctl.d/
EDIT: Just tried a debian13 LXC...