I have had the same issue deep underground in a mob spawner (y=14), making it very difficult to farm mobs (because they either dont spawn, or spontaneously combust because the game thinks they are in the sun). I have found one work-around that always works, but it is time consuming and annoying. It seems that, whatever causes the bug, it places sources of skylight in a number of air blocks in some random chunk. If you manage to find these blocks (usually anywhere that light due to sky = 15), and fill them with solid blocks, once you break those blocks the issue should be gone. You may have to fill in almost the entire affected region with solid blocks though. This light can also generate in non-full/transparent blocks like slabs, ladders, chests, glass, etc. You will have to break them, replace with a solid block, break the solid block, and then place the original block again to remove the issue.
I suggest manually filling the brightest part of the underground region with easily breakable solid blocks that you can discern from the rest of the environment. Fill it all up, break the blocks you used to fill, and all should be good.
I have had the same issue deep underground in a mob spawner (y=14), making it very difficult to farm mobs (because they either dont spawn, or spontaneously combust because the game thinks they are in the sun). I have found one work-around that always works, but it is time consuming and annoying. It seems that, whatever causes the bug, it places sources of skylight in a number of air blocks in some random chunk. If you manage to find these blocks (usually anywhere that light due to sky = 15), and fill them with solid blocks, once you break those blocks the issue should be gone. You may have to fill in almost the entire affected region with solid blocks though. This light can also generate in non-full/transparent blocks like slabs, ladders, chests, glass, etc. You will have to break them, replace with a solid block, break the solid block, and then place the original block again to remove the issue.
I suggest manually filling the brightest part of the underground region with easily breakable solid blocks that you can discern from the rest of the environment. Fill it all up, break the blocks you used to fill, and all should be good.