The bug

The /spreadplayers command does not work with entity UUIDs, it just prints the error message "Player '%s' cannot be found" (%s being replaced with the provided UUID).

How to reproduce

1. Summon an entity with a defined UUID

   /summon armor_stand ~ ~ ~ {UUIDLeast:0L,UUIDMost:0L}
   

2. Try to use the /spreadplayers command

   /spreadplayers ~ ~ 0 1 false 0-0-0-0-0
   

   → It prints the error "Player '0-0-0-0-0' cannot be found"

Code analysis

Based on 1.11.2 decompiled using MCP 9.35 rc1

The method net.minecraft.command.CommandSpreadPlayers.execute(MinecraftServer, ICommandSender, String[]) does not use the method net.minecraft.command.CommandBase.getEntityList(MinecraftServer, ICommandSender, String) to parse the entity UUID / selector / player name. Instead it partwise implements the behavior itself.