Have done some more testing and have more info.
If i go into the editor, load up the map but use a vanilla game unit it will sometimes work in the editor testing in multiplayer, but only some times.
If i put DLC units in it will never work.
If i put DLC units in, make them all non playable, and then playable again, test in the editor using multiplayer still no slots in the lobby.
However, if i do exactly the same as above except after making the units non playable (so no playable units at all) and then save the map, then make all those units playable again, save and then go to the multiplayer testing in the editor all the slots in the lobby are there to use.
Without leaving the editor, if i then load a new map and then reload the map i'd just tested it still works fine.
If i leave the editor, then re-start the editor back up and load the map which was working, it no longer has those slots available in the lobby. If i repeat the process of making all units non playable, saving, making playable again it again works in the multiplayer editor testing.
None of the above will work if i go to multiplayer in game.