Unfortunately I have noticed that the game mode editing tool is broken. It allows you to create a custom gametype, but if you try to edit it, it will not save the new version. Instead, it creates a clone of the original custom gametype with the generic “Slayer” name.
I have discovered a workaround for this error, simply by choosing “Save as” instead of “Save”, and giving the new version a similar name; however, if you delete the original custom gametype, the new gametype’s name and settings will be overwritten as a clone of the base gamemode, such as “Slayer”.