So, I used a falling block command in the most recent 1.17 snapshot, 20w46a, I was just messing around with it... So, I used the falling block command to cover over a block, a lamp, so when the lamp was turned on by redstone, you wouldn't see a lamp, just the floor somehow lit up, that was the intention. When I used it in 1.16.4, I don't know if this effects other versions as well, I used the same command again, the falling block was gray concrete, but before I turned the command on, I placed a gray concrete block in creative, where the falling block was going to be. I turned the command on, and to my surprise, the physical gray concrete block I placed was gone, so you could walk through it, the block I placed disappeared. But here's the thing, I moved the falling block/ghost block with a piston, and it worked, but now there's TWO falling blocks, NO physical blocks. So I then turned the command block off, and ONE of the falling blocks stopped. But the other block stayed, so if you wait around 30 seconds, the block drops, and you get it back. I don't understand how it works, but I do think this is a bug. Currently I'm not able to send you any screenshots or video to prove this, but if I can I will.