PC-98 mode fixed not to register certain built-in commands and
binaries to drive Z:\ that are not compatible with PC-98 mode.
MEM.EXE in particular uses IBM PC-specific techniques to gather
it's information, and is therefore incompatible with PC-98 mode.
This can be verified from the source code as MEM.EXE was evidently
borrowed from the FreeDOS project.

https://github.com/joncampbell123/dosbox-x/blob/master/CHANGELOG