I presume you followed this guide: https://docs.opnsense.org/manual/how-tos/wireguard-client.html
What I have noticed, that even though the guide lists it as not necessary for IPV4 traffic if you've configured a WG interface group, I resolved the problem you're having by adding the rule in 4(b).

Here's the weird thing: My Wireguard connectivity works fine without that rule (I can even disable it after the system has booted!), until I reboot.
Then I've got the same symptoms: cannot reach outside network.
Would be solved by manually rebooting Wireguard, or like you said, changing a little thing in the firewall.
After I explicitly added rule 4b to my system, Wireguard connectivity to the outside world now survives a reboot as well.
Would be very enlightening to have some OPNsense wizard explain to us why this is needed.
I'm out of my depth to explain why this could happen, maybe a race condition during boot.
What I have noticed, that even though the guide lists it as not necessary for IPV4 traffic if you've configured a WG interface group, I resolved the problem you're having by adding the rule in 4(b).

Here's the weird thing: My Wireguard connectivity works fine without that rule (I can even disable it after the system has booted!), until I reboot.
Then I've got the same symptoms: cannot reach outside network.
Would be solved by manually rebooting Wireguard, or like you said, changing a little thing in the firewall.
After I explicitly added rule 4b to my system, Wireguard connectivity to the outside world now survives a reboot as well.
Would be very enlightening to have some OPNsense wizard explain to us why this is needed.
I'm out of my depth to explain why this could happen, maybe a race condition during boot.