Ultimate 1MB Diagnostic ROM

I've been trying to isolate some strangeness with my U1MB install.
It seems to work perfectly fine if I use SDX on the SIDE cart, but if I use SDX on the U1MB it acts like there is only 320 KB no matter what the settings are.

Then I noticed a Diagnostic ROM setting for the OS.
When I run that the detected RAM size only shows 055296 bytes.
Does that Diagnostic ROM work?

Frank