OK, so I was curious if it might be a DHCP problem and decided to manually assign IPv4 address settings. I followed the guide here: Contents/windows-vm41.md at master · Qubes-Community/Contents · GitHub – the “After Windows Installation” section where it talks about sys-whonix (or failing sys-firewall) networking.
I was able to get it working, but not after setting the network mask to 255.255.255.255. This is also one thing that irks me in the network setup of Qubes. It doesn’t obey proper netmasks/subnetting (and Windows complains about it also). Technically, the netmask should be 255.0.0.0, because the IP Address is 10.137.0.x and the default gateway is 10.138.x.y, but that means the DNS servers will be on the same subnet as well. I should try that and see if it works anyway, but I wonder what the deal is with setting the netmask to /32.
I also noticed that the IPv6 protocol was on, and happened to tick it off to disable it whilst doing these changes, so it is /possible/ that it was stuck on doing DHCP over IPv6, which is also a common problem in Windows networking – it prefers IPv6 over IPv4. Of course, there is no DHCP server serving the IPv6 protocol, so that also makes sense why it was not pulling an IP address.
I may try to turn it back on and set IPv4 back to automatic to be sure, but these are things I tried, and it started working again, so, going to mark this as “fixed” for now. I’ll add any additional testing/comments/results that are pertinent for those that encounter this issue themselves.