That is because when sand falls onto a cactus, alike to many entities, it will be destroyed, and so it cannot place the block, leaving only the first falling sand in place, with the others getting destroyed and becoming items, which most likely also got destroyed by landing on the cactus.
Panda Draygon
That is because when sand falls onto a cactus, alike to many entities, it will be destroyed, and so it cannot place the block, leaving only the first falling sand in place, with the others getting destroyed and becoming items, which most likely also got destroyed by landing on the cactus.