Growing cactus placed on blocks other than (red) sand drops two cacti instead of breaking

If you do this command:
/setblock ~ ~-1 ~ cactus
while standing on a surface, that cactus block will continue dropping cacti forever.

The cactus block has to be on the ground, but not on sand. It still can't touch any blocks on the sides.

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Code analysis by Marcono1234 can be found in this comment.