Qubes-Whonix doesn’t “care” / “know” about the exact mechanism how its Net qube connects to the internet. It does not attempt, distinguish the different connection types. LAN or WiFi does not matter from the perspective of sys-whonix.
Therefore I do not think this issue is caused by any source code by Whonix and therefore most likely will not be able to help with this.
You could try to report a bug to Qubes upstream. As per: What to post in this Qubes-Whonix forum and what not.
Not a suitable connectivity debugging tool as per:
Troubleshooting - Whonix chapter Unsuitable Connectivity Troubleshooting Tools in Whonix wiki
I would say follow the instructions on Troubleshooting - Whonix but that seems futile since you already established that simply the Net qube setting determines functional versus broken connectivity.