While forging my save didn’t save and I think I had done at least 2 saves from what is missing.
I did see an error message as I left forge afterwards.
By any chance, had you just saved a pre-fab? I’ve noticed there is a but, where after you save a pre-fab it will no longer let you save the map you are currently forging. You must exit and come back.
To avoid losing progress on your map, simply save it first, THEN save your prefab.