The bug

The side texture for grass blocks still uses a color scheme which has remained unused since October 2010.

It hasn't been possible to actually see this texture in-game without resource packs since 2014, from my knowledge (see MC-58339). However, the bright green still has a minor effect on mipmapping.

An updated version of this texture is attached, which the current vanilla texture can be easily replaced with. Alternatively, the green pixels can just be removed from this texture outright.