I am aware that Mojang attempted to fix this bug years ago, but it still plagues the mountain that I call home. I've blocked all possible entrances for light to enter, and yet my game believes there to be a phantom skylight. This is greatly impacting my build, as I was attempting to create a dark tank for my glow squid. I am playing in 1.17, and I've had this issue present for years. Within the image provided, you can see that the client light level of the block is 3, but the sky light level is 11. I've done all I can to blot out all sources of light from the sky, but to no avail. It should be mentioned that the chunk that this occurred in generated VERY long ago, so that may have some relevance.