Wireguard issue after FW-reboot

[LineF]

Hello,

I'm running a wireguard tunnel from my smartphone to my home network.
OPNsense is the central routing instance and FW between WAN, LAN, DMZ and Guest networks.
In the WAN network is my DSL internet router (and only that one).

All (0.0.0.0/0) my traffic from my smartphone is routed through the wireguard tunnel to OPNsense.

After rebooting OPNsense I can reach from my smartphone all devices in my local networks, but no traffic is routed to (or from?) the internet.

Now the wierd thing:
When I change something of the firewall configuration, the traffic to the internet starts going being routed.
E.g. I can stop and start an service or I can enable/disable logging of a particular rule...
As soon as such an action happens traffic from and to the internet is routed through my wireguard tunnel.
Until the next reboot of OPNsense...

Any ideas what could be going on here?
How could that be debugged?
This happens since months (from the beginning I was using OPNsense - at least 12 Months)

Greetings,
Martin