Hidden fake players (ones preceded by a #) never show on the sidebar, this is by design, however they can be given a display name or number format even though it doesn't show.

Steps to reproduce

1. /scoreboard objectives add test dummy
2. /scoreboard objectives setdisplay sidebar test
3. /scoreboard players set @s test 1
4. /scoreboard players set #test test 2
5. Notice how the fake player is not displayed
6. /scoreboard players display name #test test "Test"
7. Notice how the command succeeds but still no fake player displayed in the sidebar

Expected behavior

Either one of the following:

1. The display name/number format overrides the "don't display" property of the fake player, displaying it in the sidebar
2. The command will display an error along the lines of "Hidden player scores cannot be given display values"

The first one would be useful for debugging purposes, so see what's going in with a hidden score, while the second one would communicate the current (probably intended) behavior of hidden scores properly