It's not because it's expected behavior that it is not a bug
What I expected to happen was...:
I went into a cave and expected to see nothing but pitch darkness
What actually happened was...:
I could see pretty well, enough for torches to be useless
Steps to Reproduce:
1. Put "brightness" on "bright"
2. go into a cave that should be pitch dark
3. see that it is not pitch dark, but actually pretty light
4. go on a mac to see the difference