Questions: Can I leave some of my tiles standing on top of my card or standing on the flat part of my rack.

Answer: All 13 tiles must be either on the sloped part of your hand (in your rack) or in an exposure or exposures on the flat part of your rack. If a player was a real stickler they might call one’s hand dead if the tiles are sitting on your card or even on the exposure part which is meant only for exposures. If a player is not a real stickler than you might just get a slap on the hand and a reminder that if you do that again, your hand will be called dead.

Source: Mah Jongg Made East 2020 Edition, Rules and Penalties #3.