I had the same issue since 24.1.9 and fixed it by disabling Crowdsec (when the package is installed, it is enabled by default). I did not revert dhcp6c to an older version.
Crowdsec has a cron job running every hour so it could explain why it stops after exactly 1 hour (my DHCP leases are 8h - renewed every 4h).
I did not spend time to check why it is causing WAN IPv6 to be lost. It looks unrelated but practically it works.
Crowdsec has a cron job running every hour so it could explain why it stops after exactly 1 hour (my DHCP leases are 8h - renewed every 4h).
I did not spend time to check why it is causing WAN IPv6 to be lost. It looks unrelated but practically it works.