Cactus grows and ignores 'cutter' blocks in cacti farm.

In my cacti farm, the cactus is supposed to have a block of air in the N, E, S and W direction. When it grows, the cacti is supposed to 'pop' off if there are only 3,2 or 1 air spaces. Instead, even with 3 air spaces. it does not pop off.
Only a block update causes it to pop off. Has been like this in 0.6.X.