Faint gridlike pattern underwater with smooth lighting

Found while testing the fix to MC-68129.

The bug

It seems that underwater smooth lighting, while interpolating, lights the edges between blocks more than the centers of blocks, resulting in some grid-like patterns of light.

How to reproduce

1. /fill ~ ~ ~ ~6 ~6 ~6 minecraft:white_concrete hollow
2. Enter this new cube and repair it while staying at the corner closest to you when you created it
3. /fill ~ ~ ~ ~4 ~2 ~4 water
4. Replace any one of the ceiling blocks with glowstone

Expected result

The lighting underwater would be completely smooth, especially counting the fix for MC-68129.

Actual result

There are clear discontinuities in the lighting - lines parallel to and one block away from the walls appear the brightest, and there are plenty of considerably more obvious sharp boundaries perpendicular to the walls as well.