Suddenly a black spot appeared on my iPhone 11

I have had an iPhone 11 for the last two years, and it works perfectly for my daily needs. But recently, I suddenly noticed a black spot on the bottom of the screen. Never fall the device nor water damage at all.

https://preview.redd.it/12q7ayaoe5u91.jpg?width=1200&format=pjpg&auto=webp&s=31152cce2e1bea4f5e4988144b1a432ac28cbc3c

I switched off the device for the whole night, and the spot was gone when I turned it on in the morning! It's been a week now it never appears again. But I still wondered why it had happened, and soon my display will die? I called apple care they ran the diagnostic test and said everything seemed fine. Can anyone have any clue about this incident?