This is a Block Drop bug
The big is that if you go to the edge of a block the block will not despawn. It only works if your moving though. If you just sit there the block will start to fall after a few a seconds
This bug works on 1.16.210 but it might work in lower versions. I know it works on Xbox one and Windows 10.
My videos file size is too big but the YouTuber Pizzas made a video on his second channel yawing the bug.