How to recreate the bug:
Step 1: Create a new World and summon a Wolf like in Screenshot 1 (I did not test this with other entities)
Step 2: Create a scoreboard like in Screenshot 2
Step 3 (this is optional): Put the scoreboard in the sidebar slot like in screenshot 3
Step 4: Give the Wolf a Score in the Scoreboard like in Screenshot 4
Step 5 (this is optional): Test if you can change the score like in Screenshot 5 and 6
Step 6: Save and Quit to the Titlescreen and load the World again
The Entity ID changed
Step 7: Try to change the Score of the Wolf like in Screenshot 7 and 8
It creates a new one
You can only fix this when you remove the scoreboard and add it again.