If a player stands on the top of a scaffolding block before leaving the game, then when the player logs back in, the player sometimes fall through the scaffolding and ended up spawning at the wrong y level.

This seems to happen only when the player is at least 19 chunks away from the world spawn. It affects players in any gamemodes other than spectator mode. Also this bug occurs less frequently in superflat world, but it almost always happen in normal world.