Just Updated to the newest snapshot and random portions of grass blocks have disappeared in chunks that had been generated before updating. Newly generated chunks were fine however. This world was previously on 1.7.10 and was originally generated in MC alpha 1.1_02 I believe (the update just before the nether update). I suspect it is related to MC-50998 as, in previous snapshots, the same parts of grass had missing textures.

Edit: having done some research, I think that the reason these blocks are invisible is because older worlds had scrambled metadata for blocks (think leaf blocks after the beta 1.3 update). because of this, the game can't find the correct texture for the grass block and should render them as black and purple checkered blocks. However while that is a problem within itself, a new bug has arisen causing the blocks to instead be rendered invisible, similar to loading up a modded world in vanilla. Thanks to MC-69646 for some of this info.