Gateway failover group and default route often not reverting to primary gateway

[Maurice]

I'm currently experiencing issues with a straightforward dual WAN system: Two gateways, both marked upstream, the primary has priority 1, the secondary priority 2. Gateway monitoring is enabled for both. Default gateway switching is enabled globally. There's also a gateway group where the primary gateway is in tier 1 and the secondary is in tier 2. Trigger level is member down.

When the primary gateway fails, failover to the secondary works fine, both for the default route as well as for the policy rules using the gateway group.

Not so when the primary gateway comes back online. It reliably gets marked as active in System: Gateways: Configuration, but the default route (System: Routes: Status) as well as the policy rules (Firewall: Diagnostics: Statistics: rules) frequently (but not always) stick to the secondary gateway indefinitely.

I remember having had similar issues in the past and that significant improvements have been made in this context. But apparently this hasn't been truly resolved for all scenarios. Are there known open issues in this area? Can't find anything obvious on GitHub at the moment.

The reason why I'm currently noticing this is the primary WAN having more frequent outages, so this might not be a recently introduced issue.

Primary WAN: DHCPv6, request prefix only, interface address configured via optional prefix ID / interface ID setting
Secondary WAN: SLAAC

Cheers
Maurice

[Maurice]

New observation:
Sometimes the system's default route reverts to the primary gateway correctly.
But the policy rules (pass in quick on .. route-to (..) inet6 ..) stick to the secondary gateway.
Clicking Apply in System: Gateways: Configuration fixes the policy rules.