I experienced this when attempting to harvest cactus and wheat seeds from grass. Almost half the wheat seeds that were spawned while harvesting grass appeared to disappear as they hit the ground, and I was not able to pick these seeds up. This also happened when harvesting cactus, but I am not as confident of this bug, as the missing blocks may have all just landed on top of the remaining bottom portion of cactus (is this itself a bug? I don't know)