When you say it's not possible to edit the Default network, I think you meant, "The inability to edit the Default network is the bug you're experiencing."
We've configured dozens of GX20s where we edited the IP address for the Default VLAN. This is no longer possible.
Using your workaround as an example: Set the Default to be a Dummy-LAN . . . ok . . . does that just mean you turned off DHCP? I mean, that VLAN still exists with the default IP address of 192.168.128.1, which arguably doesn't make it a "dummy" - instead it means I can never use that IP scheme.
OK, using the workaround you suggest: I create a new VLAN "Test" and assign it an IP address, say 192.168.100.1.
To save it, as you said, I have to click Cancel, then Save. (I think everyone would agree that's awful & non-intuitive, but it is what it is).
So, I have a new VLAN "Test" . . . with the IP scheme I want. I now create an access port so that devices get DHCP from this new VLAN. Is this the new best practice for configuring?
This is incredibly frustrating.