There is this bug wherein light originated in locations above y: 31 is originally 0.
This means it gets to light level 0 as you go higher.

Light from below will still propagate to y: 31 and above. Torches still work but don't update the block lighting. Putting a torch and breaking it will mean the same result. No light.

Reloading the world also doesn't work.

Please fix this bug. It is going to be disturbing especially for map makers.