-> /scoreboard objectives setdisplay sidebar A
-> /scoreboard players set player A 1
Now the objective A is displayed on the sidebar and player has a score of 1.
-> /scoreboard players reset * B
Now A isn't displayed on the sidebar anymore.
-> /scoreboard players set player A 2
Now you can see the objetive A on the sidebar again.
So resetting players on scoreboard objective B causes the sidebar for the displayed objective (A) to vanish. Update any score on A and the sidebar reappears.
Alternatively, setup these initial scoreboards:
Now, resetting Player Two's score on test2 should NOT clear them on test1's scoreboard display, but it does...
TL;DR
If you reset a player's score on one objective, It resets every player with that name on everyone's setdisplays, even if it's displaying a different objective.