Falling Blocks That Are Skulls Don't Show Falling Animation

Concerning the skull as falling_block that doesn't render, that's MC-64634.
But can you elaborate about the second part of your issue?

The second part is that when they fall sometimes they will destroy the block under them and replace it with itself

Cannot reproduce then, unless you provide more information about the type of block they replace, what exact command you are using to summon the falling_block, how high the skulls need to be summoned, and how often it's happening.