When you update a flat or amplified world that was previously on Minecraft Xbox one edition and you go below y negative 48 and /fill block in and leave the game, the. Join back in and destroy the blocks using /fill the. Leave and join back again, there are weird block shapes left over and are difficult to remove. You cannot remove them with /filled tnt however you can with tnt you placed and I think you can break it with your fist too but I’m not doing allat lol