I was exploring the surface with Elytra and fireworks, then I found this, a line of chunks with a light level of 0 (Mobs spawn in) and some other chunks around with the same problem

Placing a torch inside the chunk is like placing a toch inside a cave, and remove it will instantly set the light level in this chunk back to 0

but placing a torch outside the chunk will actualise the light level and set it back to normal

Coords: X = -3621 Z = 2582