The bug

After setting a new world spawn point, it is impossible to set blocks or summon entities there until the spawn point has been visited by a player. If the area is visited and then left, then it is possible to /setblock and /summon to those locations.

How to reproduce

1. Create Superflat world
2. /tp 0 60 0
3. /setworldspawn 10000 60 10000
4. /setblock 10000 60 10000 stone
   → "Cannot place block outside of the world" if the region has never been loaded or "The block couldn't be placed" if it has
5. /tp 10000 60 10000
6. /tp 0 60 0
7. /setblock 10000 60 10000 stone
   → Block placed

/setworldspawn should actively move spawn chunks to the new location and load/generate it; as it is currently working, the command is only useful for avoiding using an NBT editor for moving the world spawn coordinates.