I'm creating a separate report for this bug, as I originally ascribed it as a symptom of MCPE-186374 but later found out that changing your render distance doesn't affect it, unlike both the spruce and birch leaves reported there (and also the fact that those are semi-transparent blocks, meanwhile grass blocks are opaque). Also, it's worth noting that with these grass blocks, only the top texture is affected.

Steps to reproduce:

1. Make sure you have "smooth lighting" disabled under Settings > Video
2. Locate any roofed forest biome.

Observed results:
All the grass blocks within that biome will be pitch black.

Expected results:
They shouldn't be black.