Steps to reproduce

1. Place sand.
2. On the sand, place cactus.
3. On the cactus, place some blocks.

Expected result

The cactus breaks.

Observed result

The cactus does not break.

You can place blocks on a cactus. normally, you cannot place any blocks sides of cactus and upper side of cactus, but you can place blocks on upper side of cactus.