Falling blocks not falling when placed directly above certain blocks

"Falling blocks" refers to sand, red sand, and gravel.

Blocks that will prevent falling when directly underneath the placed falling block are: skulls, torches, slabs, switches, redstone, etc. – basically anything that doesn't have a valid top surface for falling blocks to land on

To reproduce, simply place a block without a valid top surface to catch the falling block when it's dropped from a higher point, then place a falling block directly on top. Rather than falling, the block will simply sit where it was placed