At height 30(bedrock is 1 or 0) on creative mode on flat when you leave world(you dont have to be at height 30 atm) change it to survival mode(does nothing) rejoin then leave then change to survival mode then change to creative mode ... when you go into a block at that height that you placed you can go through every block (after quitting game then rejoining) until you die

THIS ONLY WORKS SOMETIMES