When using the command /spreadplayers it will not tp you somewhere on the ground. Instead it will tp you to y-level 255 and send you falling to your death. The command I have used is

/spreadplayers 0 0 300 3000 @s

Other than the y-level problem it works just fine. I have done this on a private world and on a realm and it does the same thing in both. I have tried turning off an on experimental and education edition but nothing has fixed it. The only thing I have found to fix it is if the chunks are already loaded, then it puts you on the ground. but with the numbers as large as I am using that rarely happens.