OK, nothing to do with the port (there was nothing listening on that port, and writing a tiny bit of Python code, I had no problem binding to and listening on port 5350).
I hopped over to my older Windows 10 system and found that it also had the same issue (and this used to work just fine on version 1.8).
One of the things they both have in common is that I have Hyper-V configured and there are various virtual networks configured. This turned out to be the issue.
On the Windows 10 box, I deleted the private networks (172.16.0.0/24 and 172.16.1.0/24), restarted Roon, and all of the local audio devices are now available.
So. this is a bug in the 2.0 client. It needs to ignore private networks, or, at the least, if it’s going to try to bind every interface, don’t fail if at least one succeeds. Instead, it seems that the attempts to bind on these private interfaces caused the entire setup to fail.
Tim