Glad everything is working as you want now!
this is not unexpected, actually. It is a quirk of the way that the WG interface comes up -- it effectively replaces the existing routing table and does not restore it when the interface goes down. In order to resolve this, you can do one of two things:
- put a gateway metric value on your WAN
- use the hotplug.d scripts to restart the WAN interface when WG goes down.
Meanwhile...
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