Hi,
I found the same thing on my 2019 server and 2012 server, there are not such a Show clear logon background setting.
It seems there are difference for the logon admx file on the server and the clients.
When i copied the logon admx and adml file from the clients and replaced the ones on the servers, the setting displayed on the server .
In my environment , the admx central store is used.
If the central store is not used in your environment, the admx file is just under c:\windows\PolicyDefinitions
Best Regards,