I made a bunch of quick maps with over 50 props each and named them 1-50 (then 51-70 when nothing happened) and did not get it.
So it appears it doesn’t work if you “save as” or if you do additional versions of an already edited map. Best option is to open an official 343 map in forge, place 50 props (I used sandbags) and spam save. Also if you leave your custom map and reload it progress will stop progressing and you have to start again with a clean 343 map.