[SORRY FOR THE MUSIC IN THE VIDEO]
As you can see, there is a visual bug in which we can see non-visual zone of the mine.

In the first photo, at the end, we can see something dark, it isn't a non-illuminated zone, it's a visual bug.
In the second photo, we see more of the bug.
In the third, we see the lava's pool.
In the fourth, the bug disappear and all is normal.
The fifth and the sixth we see the rock wall where the bug was.
Finally, the last photo, is where we were looking at when we saw the bug.

It's a simple visual bug like the common light's bugs in which it disappear when you interact with it by breaking a block, or putting a torch (this isn't the case)

1. Go into a mine, that one was a large one (if that helps.. :S)
2. Find a place that doesn't have a lava pool or something that is like a light and put a torch near the point in which the bug happen. (in the middle of that torch and you, that is also illuminated by a torch, there isn't any illumination if that helps as u can see in the photos)
3. You have to put your player in a long range and faced foward to that large distance with a distance of 16 blocks in diagonal. In the 17th block the bug happened.

I hope that all the photos and the 2 video would be usefull for you!