When you generate a new world, the photograph it takes for the world selection screen sometimes takes itself before chunks are fully rendered.

It might be caused by generating a world when the game is lagging a lot, such as an amplified world. 

Update: I bet that is what is causing it because I generated an amplified world and it happened

1.17 Update: It didn't happen for a few versions, so I assumed it was fixed. However, in 1.17-rc2, it occurred again. Default world, not amplified.