I was instamining an area out one chunk at a time and I saw the smooth lighting wasn't looking so smooth, so I updated it to see if it would go away and it did. I don't know if it was because I was instamining or that it was on a chunk border or some combination.

It was happening pretty consistently too. Like I dug 2 blocks down and it happened again.

Wait I have some clue of what is causing it, I am digging a perimeter around the inner part of the chunk so I can go around it and dig the inner part out (I'll provide pictures), maybe that is causing it.