This happens in the newest 21w10a snapshot and possibly older versions. As I am running around the world sometimes the game night cycle stops working in a group of chunks and the game is acting like its always night there. mobs spawn in the day time in that black section and they don't burn unless they leave it.

IT IS FIXED WHEN RELOADING THE WORLD

Video of the bug: https://youtu.be/wSTBeC7rAqc