There is nothing wrong with this
Try open up a newly-fresh map (.w3m) and edit some fields in Object Editor and press CTRL + F9 for test map
Note that a message window will be prompted saying that
"This map is no longer a melee map because:"
It will state the reason and because of this, the file type is changed instantly from .w3m to .w3x because it's NOT anymore, a MELEE MAP