Double tall plants only generate with bottom half in mega taiga

I noticed while I was exploring a new world I made in pre-1 that in the mega taiga biome, some double tall plants only generated with the bottom part. Other biomes not checked. Didn't notice in plains. 

Expected: Double tall grass/ferns fully generate 2 high as usual

What Happened: Only the bottom half are generating it seems, but every once and a while one has both tall.

Specific seed for the world in the first picture: 3816324406572942064. Coordinates in pic