I'm pretty sure I've screwed around with this in the past. I've compared the .txt map files side-by-side and was never able to figure out exactly what allows Map Designer maps to auto-download but not real Galaxy Forge custom maps.
Indeed. Seems that the Map Designer folder works with any random map; whether it be one made ingame or via Notepad.
Further, I advise that the Devs add custom map sharing to Sins of a Solar Empire: Rebellion.