The bug

The maxRange parameter of /spreadplayers has to be greater than or equal to 1 despite not being centered on the block grid. This makes it difficult to consistently limit the amount of valid locations to less than 9.

This does not apply to 1.12.2 since /spreadplayers run apparently centered on the block grid.

How to reproduce

1. Use the following command

   /execute as @s at @s align xz run teleport @s ~0.1 ~ ~0.1
   

2. Spawn multiple entities around you, for example armor stands
3. Use the following /spreadplayers command

   /spreadplayers ~ ~ 0.0 1 false @e[type=!player]
   

   → The entities are teleported to 9 possible blocks, but how would you define that only the closest 4 blocks should be used?