The Bug

The scoreboard players operation command appears to apply operations in a loop as opposed to concurrently.

To reproduce:

1. Create a new empty world (void of entities) and create a new scoreboard objective test
2. spawn some entities
3. Set the test score of all entities to 1
4. Run /scoreboard players operation @e test += @e[limit=1] test
5. Run /execute as @e run scoreboard players list @s to see the scores

What I expected was that all scores would be 2, since that was the score of the first entity. However, every score apart from the player will appear as 3.

Now, this might be intended, however my hypothesis is this is an unintended side effect of scoreboard players operation allowing selectors that can return more than one entity.