Describe the bug

Sky goes black when roof over head, or in what game considers "cave"

Describe how your minecraft is set up

Describe how to reproduce the bug

Go in any hole in the wall of a mountain or house roof and have CAVE FOG enabled. Seems to be related with how the cave fog is determined.

this is intended. i blot out the sky so if a chunk is unloaded whilst inside a cave, the bright sky does not create a giant shining bright spot and make your eyes go blind. you can disable cave fog entirely in shader settings -> fog settings -> cave fog

but unfortunately, accurately detecting when you're actually in a cave is very difficult, and there is no good way to do it. iris/optifine do not give much information about the world, and i cant "just get it" either.
the cave fog uses a basic test, it just waits for about 10 seconds if you're in a sky light level of zero before activating, and deactivates when you go to a higher light level... sadly light level zero exists in houses, under floating islands, large cliff overhangs, ect. so it doesnt exactly only happen in caves only.