2-block flowers, ferns, grass placed in 1.7 release are uprooted in recent snapshots

User-placed 2-block flowers, ferns, and grass appear without their top blocks and uproot themselves in snapshots since 14w29a (and perhaps earlier). Some Flowers placed from certain positions are unaffected. To reproduce:
1) create a new world in 1.7 (1.7.4 and 1.7.10 tested)
2) stand in a grassy area and place 2-block high flowers, etc, in each of the four compass directions (i.e. stand in one place and make a rectangle of flowers around the player).
3) save game, then load in a recent snapshot
4) top block of most of the flowers will have disappeared; many of the affected flowers will then drop themselves after a short time. However, flowers planted while facing in one of the four directions (North in my tests) will be unaffected.

I'm re-submitting this since the description for the existing bug does not make clear the impact to the forthcoming 1.8 release, and implies that it is merely a rendering problem.