While update the pre-release 3, the bottom face changed after this model.

cactus model 1: has no bottom face

cactus model 2: slightly changes to flat block

cactus model 3: normal cacti

cactus model 4 nether update first release: still no bottom face

cactus model 5 nether update: slightly changes and the top texture is slightly darker

cactus model 6 nether update pre-release: no dirt; cacti changes bottom and top face