I ran into this last night, and thought it was pretty weird. Basically, it was night outside, but when I was in a cave looking out, it looked like it was day. Then, as I exited the cave, everything suddenly changed to night. The reverse happens when I back into the cave. Also, as you can see in the video, there is some weird flickering of the mountains in the distance. I don't know if this happens in every cave, as I haven't noticed it otherwise.