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:
- Create 2 new scores
- Display the first score
- Do an operation (notice that the sidebar didn't change)
- Update sidebar (notice that now your score appeared on the sidebar)