Hi, I have basically the same problem described here (https://forum.opnsense.org/index.php?topic=41723.0) but I'm on 25.7.3
I get a static IPv4 address and IPv6 prefix from my provider.
My WAN settings:
IPv4 Configuration Type: PPPoE
IPv6 Configuration Type: DHCPv6
Request prefix only & Send prefix hint are both checked
Clients are able to connect to websites via IPv6 and my servers are reachable via IPv6 from the internet. Also IPv6 Ping via Interfaces > Diagnotics is working.
But if I want to Enable Gateway Monitoring the gateway is "Offline" and the Gateway monitor isn't starting with this error in the log:
/usr/local/etc/rc.syshook.d/monitor/20-recover: The required WAN_DHCP6 IPv6 interface address could not be found, skipping.
Routes are there:
ipv6 default fe80::ded2:fcff:fe24:dd60%pppoe0 UGS NaN 1492 pppoe0 WAN
ipv6 2606:4700:4700::1111 fe80::ded2:fcff:fe24:dd60%pppoe0 UGHS NaN 1492 pppoe0 WAN
I also tried the give the gateway an IPv6 address from within my prefix without luck. If I uncheck "Request prefix only" the gateway still gets an fe80 address.
Address from fe80::/10 for a gateway is perfectly normal, in many environments considered best practice.
Ask your ISP if their gateway is supposed to be "pingable" and if it's not ask them if they can give you another address that is supposed to be always on for monitoring purposes.