If you set up a tempo marking like this:
and then in Layout Settings uncheck this setting for a part:
you get this:
I’d argue probably nobody wants that result from that combination of settings. And I don’t find a way to have parenthesized M.M. in the score if I want to hide them from the parts.
I guess the layout setting cannot change the property for parts only because it’s a global setting. But maybe it could check for the presence of parentheses as well as M.M. and just hide them together?