Check your IPv6 routes, especially if you see 2 default routes on your WAN interface, with me, every time that my PPPoE connection restarts, I get a weird default route without a link-local address as nexthop the valid one should have a fe80:... as a nexthop/gateway not your interface, check if you see this as well.
Flags: D - dynamic; X - disabled, I - inactive, A - active;
c - connect, s - static, r - rip, b - bgp, o - ospf, d - dhcp, v - vpn, m - mode>
H - hw-offloaded; + - ecmp
# DST-ADDRESS GATEWAY DISTANCE
DAv + ::/0 PPPoEv6 1
DAd + ::/0 fe80::5a00:bbff:fe0b:... 1