When you do an operation between 2 'empty'^* scores the first score will receive a score of 0, even though you get an error message: "No <name> score for <player> found". This score won't show up on the sidebar until you reopen the world or update the display slot.
^*: when you created a score but you didn't set any value for an entity

Steps to reproduce:

1. Create 2 new scores

   /scoreboard objectives add test1 dummy
   /scoreboard objectives add test2 dummy

2. Display the first score

   /scoreboard objectives setdisplay sidebar test1

3. Do an operation (notice that the sidebar didn't change)

   /scoreboard players operation @p test1 = @p test2

4. Update sidebar (notice that now your score appeared on the sidebar)

   /scoreboard objectives setdisplay sidebar
   /scoreboard objectives setdisplay sidebar test1