Thank you for the update!
Latest Enhanced Default still has that white void plane with OptiFine 1.15.2 HD U G1 pre16 on Forge 31.1.75, although I am sure you are aware of this. Was this confirmed as being at the hand of Optifine? From what I can tell, it seems to be related to the moon.
As well, could I recommend that dead_bush be added to block.10059, and that 10059.0 be added to grass_shadows? For consistency sake, being that double plants are in there, seems a bit strange that things like flowers are still causing shadows.