The media creation tool is broken in that sense from the beginning and wasn't ever been fixed. The same problem occurs without UAC and a plain, non-admin executing media creation tool elevated with providing admin credentials. It starts and aborts at some point telling one needs to be admin, whcih is stupid of course, because the tool was started as elevated admin and that approach works for all othger software. It's somewhat likely that the tool checks additional things like interactive sessions and their current user context or something stupid like that. MS itself doesn't seem to understand workflows and their own operating system sometimes... :-)
It's very likely that you are able to work around that problem by enabling the built-in admin account, logging in with that and run the media creation tool then.