Blocks in caves are receiving full sky light. It seems to be centered near a chunk border. There are full blocks above it.
I've only encountered it once, and y<16, but it seems very localized and only that area.
It does not go away with block updates, and breaking blocks under light level 15 blocks does not make the block under it have a light level of 15.
It does not spread through chunk boundaries, although breaking blocks at the boundary does update it and make light go through, but this is reset with unlogging.