SUMMARY
When you have a WireGuard connection in the "WiFi & Networks" panel of KDE and select your Ethernet to automatically connect to that WireGuard VPN, upon next reboot, the Ethernet connection can't connect to the network at all, meanwhile the WireGuard connection already seems to be established (which should be impossible if the Ethernet hasn't established the connection yet). The only way to solve this is by manually disconnect from the WireGuard VPN first, then edit the Ethernet, uncheck the "Auto connect to VPN" checkbox and apply the settings. After that the Ethernet adapter will immediately work again and connect to the network. This only affects WireGuard VPNs. OpenVPN configs work without issues.

I can only test this with Ethernet. I do not know if this affects wireless adapter aswell, because I don't have WiFi.

STEPS TO REPRODUCE
1. Open the settings and go to "WiFi & Networks"
2. Have a WireGuard VPN configuration in your "WiFi & Networks" settings panel
3. Select your Ethernet adapter, go to "General" and check "Auto connect with VPN" and select the WireGuard VPN
4. Reboot
5. Your computer will now have trouble connecting to your network
6. Open the settings and go to "WiFi & Networks" again
7. Disconnect from the WireGuard VPN first
8. Uncheck "Auto connect with VPN" and click apply
9. Your Ethernet will now connect to your network without issues

OBSERVED RESULT
Ethernet connection stops working on "Auto connect with VPN" for WireGuard VPN configs after reboot

EXPECTED RESULT
Ethernet should connect without any issues, just like with OpenVPN

SOFTWARE/OS VERSIONS
Operating System: Arch Linux 
KDE Plasma Version: 6.4.3
KDE Frameworks Version: 6.16.0
Qt Version: 6.9.1
Kernel Version: 6.15.8-zen1-2-zen (64-bit)
Graphics Platform: Wayland