Vines orienting improperly in inside corners

Previously listed as MC-50729. Bug still present in 14w18a. Not fixed. Affects snapshots 14w10a and later up to current snapshot release.

Columns of vines growing down into (not placed into) the left-hand half of an inside corner will render out in the air when vines are already present on the right-hand side of the same inside corner.

To reproduce:

In creative, place vines on both sides of an inside corner so that they will grow downwards and into the same block of air. Let some time pass. Observe that vines on the left side will pop out into the air instead of growing naturally. (See image.)

Reverting to 14w08a reverses the effect, causing vines to render normally.