Appliance DNS settings fail to apply

Version 3.8.0.35; in the appliance config page, I select 'Set Networking Parameters', choose the connection (there's only one: ens160). I tick the box to 'Register with DNS'. Hostname is populated. I enter the Domain Name and click to apply.

I immediately get the error, "Failure occurred during network interface configuration: Failed to apply DNS settings for ens160. Response from server: update failed: REFUSED."

The config is static (so not DHCP). If I leave the domain field blank, it errors as expected due to the missing required domain info.

What gives?