The bug

The cracking animation is darker on the sides of grass blocks than it is on the top and bottom, and darker than on the sides of other blocks.

How to reproduce

1. Punch grass

Expected results

The side cracking wouldn't be darker than the top.

Actual results

It is.

How to fix

This is without a doubt due to the sides of grass blocks using two textures (a base and an overlay), making this issue effectively the same as MC-166101 for blocks.

For cutout-rendered blocks (of which minecraft:grass_block is one), this issue could be mitigated by not drawing the crack animation over pixels with 0 alpha. This would fix MC-142156, and would largely mitigate this issue with grass blocks (the darker cracks would be localized to only where the grass overlay is, rather than the entire side face). If those pixels were removed from the non-tinted grass block underlay area, the issue would be fixed entirely.