-> /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.