Never tried to edit a ME that is inside one of the vanilla Masters ... but I suspect you are talking about a Master File that you created, Ducey?
I can't really test that, if that's the case, as I don't have any master-files I created.
There doesn't seem to be an obvious reason why a master would behave differently than an ESP, in this regard; but I guess they do behave different with some other things, so this may be just one more? Sorry I can't be more definate ...