IPv6 Router Advertisement seems to fail

Hello

I have a strange issue with my UTM. For now reason it seems like the Sophos stops broadcasting it's router role for IPv6. It simply stops routing IPv6 Prefix and even with a static assignment it is not routing any IPv6 Traffic anymore.

However: The problem seems resolved when I turn router advertisement off and on again (on any interface). But the problem reappears after about 4-5 days without any other changes to the UTM and it feels like this should not happen.

This happens with and without IPv6 DHCP.