As a matter of interest, i kept getting disconnected multiple time last night, my ONT would stop responding and the service and ethernet light would go off. My DHCP client session said interface not found (ONT Ethernet interface is shut when i test it). If i leave it, it comes back about every 5 - 10 minutes or so and reconnects but drops again shortly thereafter. More like a port shut due to a loop or spanning tree kicking in. This is the ONT ethernet interface connecting to my routers wan interface that becomes unavailable, not the routers wan interface. If i remove IPv6, the port stayed up.
Not sure if this issue was due to me adding 2 /64 IP pools from my assigned /62 onto my 2 of my vlans, though it ran for over a day just fine. To get it running again, i forced a new IP prefix via releasing the current IPv6 about 3 times before it remained stable. I only have one /64 pool assigned currently to one vlan, will add another later to test, too much work this morning to fiddle.