Seems that gravel physics is flaking out like how minecarts can sometimes and end up being pushed together as one object inside each other. Found this out by being in a old abandoned mine and broken some cobwebs only to find gravel fall and stack together and then keep on falling then break as it hits the ground under the cobwebs.

Very possible this is 'physics' related bug to same issues with minecarts that stack up physically together....

Repoduction: Any game mode: Place 2 cobwebs ontop of each other, place 2 or more gravel blocks above that. Break top cobweb block. Gravel will then fall and stack up together as one block and keep moving thru the last cobweb till hits the ground, then breaks to the ground as items to be picked up.