After loading a save, some areas are always dark even if there is a torch nearby.
In the affected area, the block light remains 0 even if I just stand above a sea lantern as we see from the attachment.
It seems this problem only occurs in 1.9.
In 1.8, it is fine.
This problem can be temporarily fixed by changing the light manually such as placing a torch in the affected area.
By doing this, ALL the affected areas become normal until next time I load the save.