PPPoE IPv6 gateway always marked as offline if gateway monitoring is enabled

[niabot]

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Describe the bug

I added an additional WAN interface using PPPoE and VLAN 7 (Deutsche Telekom) to my multi wan setup. The interface (tier 1) works fine using IPv4 and IPv6. But as soon as i enable gateway monitoring the DHCPv6 interface is marked as offline, and tier 2 fallback is activated.

To Reproduce

Steps to reproduce the behavior:

1. I followed the instructions of https://docs.opnsense.org/manual/how-tos/ipv6_dsl.html to add an additional WAN gateway
2. I added this gateway to my gateway groups for IPv4 and IPv6, making it tier 1 and the other tier 2 and tier 3 respectively.
3. I added a unique monitor IP 2606:4700:4700::1111 (Cloudflare DNS) and disabled "Disable Gateway Monitoring"

Expected behavior

Since the gateway is working fine, i would expect that the monitoring would also work, but it doesn't.

Describe alternatives you considered

I disabled gateway monitoring for this gateway. Automatic fallback to tier 2 for IPv6 will not work.

Screenshots

Gateway Monitoring disabled

Gateway Monitoring enabled

Interface

Relevant log files

Not sure which log files i should provide.

Environment

OPNsense 24.1.6-amd64