When loading a world or reloading a chunk, sometimes the ground doesn't illuminate like it should, displaying as completely unlit, despite the presence of light blocks immediately next to or within the darkened area.

In the screenshot provided, you can see the dark area should be illuminated by the jack-o'-lanterns next to and within it, yet the ground remains dark.

The area updates its lighting when you place a light source or place and remove a solid block.