While playing in a multiplayer world with one of my friends, he discovered that concrete (sand) and sand, when falling over a cobblestone wall, they acted like a non solid block. The blocks giggle a little bit.
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This issue duplicates MC-90523.
That is the correct behavior for falling blocks. Cobblestone walls and other fences are 1.5 blocks tall so that you can't jump over them, but this means that their hitbox extends above the block they are in. The falling block (which is an entity while it is falling) falls down and rests on the taller hitbox of the fence and stays there waiting for you to break the wall so that it can finish falling to the ground and become a block again. It can never reach the y-level it needs to return to being a block because of the taller hitbox of the fence.