Happy new year, happy new savegame! 
Savegame now works as it should. Just press “P” to save the whole map.
Pre-building “spawn volumes” checks each block if it’s already stored in savegame, except “random volume”, which stops altogether as soon as it finds one, this is because a new random number might be higher than the old and thus might fill the volume more each save.
Preview meshes might show double meshes in editor, but will work in-game
If you build for example a stair using “SpawnVolumeStair”, destroy it in-game and save; next time you load the map, the volume will “fix it”, unless you deactivate it or delete it