If we consider that the "show on desktop" feature is a basic one like it's only supposed to show the editor on the taskbar (on the desktop ?), then it's probably a bug since with that in mind, you don't want anyone to access something related to this instance of mirc.
However, if you see the feature like the editor is sort of another program, a standalone editor, then you might want to be able to use the editor whatever the mirc's state (locked).
Since it doesn't really make sense to lock mIRC and still want to be able to use the editor, I think too that this should be "fixed", it's unlikely that the behavior is intentional, rather a case that never happened so far