The new user message is supposed to be in the Template:Welcome page, not the MediaWiki:Newusermessage-template page.
MediaWiki:Newusermessage-template is simply supposed to contain the name of the New User Message template page, it is not supposed to be the actual New User Message page.
I have already copied the New User Message to Template:Welcome. All that remains to be done is to delete the MediaWiki:Newusermessage-template page.
For more information, see the 'In-wiki configuration' section of [1] Pyrrhus (talk) 16:41, 12 February 2019 (EST)