In at least 2 different worlds (both superflat, if that matters), I have found sunlight underground despite being completely closed off from the sun. The problem can be partially alleviated by replacing stone with another block, but not all the light goes away. Opening the f3 debug menu shows that the light is counted as sunlight, and it lacks the tint from other light sources.

The first world I noticed this in, I believe I was in 1.13pre2. I dug underground to build a cave and used /fill to carve it out, but found that the inside was completely lit up. The bug eventually resolved itself when I covered the walls and replaced the floor, but it was still odd.

The second time was in 1.13pre6, when I opened a world last loaded in one of the snapshots and found that the lighting had messed itself up. I had only a 2x2 shaft of light filtering in to the cavern, yet the entire thing was lit up as though I had night vision on. Digging into a wall showed the sunlight levels decrease (I turned on f3 as soon as I saw what was happening), but everywhere else had sunlight at 15 unless I dug out and replaced the block directly below me. I loaded the world again today (in 1.13) to see if the bug had been resolved, but found that the aforementioned tunnel had become lit up like the rest. I managed to remove some of the light by using /fill to replace the walls with another block, but some of the light still persisted.