Similar to, but not the same as MC-26203. Fixing MC-26203 might probably also fix this bug.

Players USED to assumed to be 0 when you added a scoreboard objective, but now they're assumed to be "nothing", until you set a number for them on a scoreboard. If you use "scoreboard players operation" on them, and their number doesn't change , they should still be nothing. But now they're actually tracked as 0. But aren't actually 0. They're actually nothing AND 0. They don't show up on setdisplay sidebar as 0, but CAN be tracked by a commandblock as a player set to 0.

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Add initial scoreboard stuff:

/scoreboard objectives add test dummy
/scoreboard objectives setdisplay sidebar test

Now, this next command doesn't work, and shouldn't work because the player is not yet on the scoreboard:

/say @a[score_test=0]

But using an operation that doesn't change the player's number manages to work at making them 0, and detectable by other command blocks as such:

/scoreboard players operation @p test += @p test
/say @a[score_test=0]

But if you notice, you don't show up on the sidebar with the number 0. So you're 0 that can be tracked by other command blocks, but you're not able to be picked up and displayed by the setdisplay.

A very quantum bug indeed...