When cutting/mining/breaking/dropping blocks too close to a cactus block it will absorb the block and it is no longer in existence. This also affects cactus blocks themselves when you cut the top blocks from a base block of a cactus plant. I have seen the same happen to bamboo blocks when you cut one down too close to a cactus plant.
I have also tested this by dropping blocks (dirt and stone since I really didn't want to keep wasting resources in a survival game) onto a cactus block using the Q key and it is a consistent issue.
This issue has existed for some time and in multiple snapshots/versions but I have been unable to find a report about cactuses that mentions this.
It is a particularly vexing issue since I quite often use cactuses and bamboo as outer defenses beyond village walls and losing dropped blocks as I farm these defenses to increase their numbers is annoying.
The wiki: "Cacti destroy any items that come into contact with them, including cactus blocks harvested above them."