The bug

The scoreboard and commands related to it do not handle entity UUIDs correctly. They treat them like regular player / fake names instead of matching the entity with the respective UUID.

Affected commands

• /scoreboard
  • /scoreboard players list
  • /scoreboard players set
  • /scoreboard players get
  • /scoreboard players add
  • /scoreboard players remove
  • /scoreboard players reset
  • /scoreboard players enable
  • /scoreboard players operation
• /team
  • /team join
  • /team leave
• /execute
  • /execute if score ... matches
  • /execute if score ... = ... (all operators)
  • /execute store ... score

Reproduction steps

1. Add a scoreboard objective

   /scoreboard objectives add test dummy
   

2. Summon an entity with a specific UUID

   /summon armor_stand ~ ~ ~ {UUID:[I;0,0,0,0]}
   

3. Set a score for the entity using a selector

   /scoreboard players set @e[type=armor_stand,limit=1] test 1
   

4. Set a score for the entity using the UUID

   /scoreboard players set 0-0-0-0-0 test 2
   

5. List the scores

   /scoreboard players list
   

   There are two entries instead of only one for the entity

Code analysis

1.16.2, Mojang names

net.minecraft.commands.arguments.ScoreHolderArgument.parse(StringReader) only parses the argument as selector (which also covers UUIDs) if the value starts with @ (which is not the case for UUIDs). Otherwise it treats it as literal name.