They also appear darkened or black whilst being held in first person.

Id guess it has something to do with Complimentary Reimagined's seasons setting conflicting.

List of blocks:
- Oak Leaves
- Dark Oak Leaves
- Jungle Leaves
- Acacia Leaves
- Lily Pad
- Tall Grass
- Fern
- Short Grass
- Vines

Below are before and after comparisons:

Also example of held items:

I also found that the top face of grass blocks also are invisible, and discolored when being held, though all the other faces of the block are normal.