i found a bug that breaks your world when you set your world spawn to where you want it to be on old world sizes. as seen by the screenshots below, the blank map still show the part of the map that you can not go to and where you spawn will be invisible; you can still pick block and mine. make sure you have copys of worlds before changing the spawn point as doing this you will lose parts of the map. one of the sceenshots is the map before it broke and the others are after. be careful (if you still use old size maps) i use bedrock and xbox one and i tested on xbox an it breaks as well, sceenshots from PC
UPDATE:
after some testing, the world spawn can only be the Original spawn point not the newly set spawn point. so i say it has to do with game issue. so if you have this on one of your maps try and find the first spawn point or get the seed and make a new world, see where you spawn, go back to your first map and reset your spawn point to that spot