For reasons unknown to me, on a singleplayer world, saplings are not progressing in growth stages (0 to 1, or 1 to grown) after waiting an extended period of time after placing. Upon reloading the world, the game will realize that whatever growth timer exists has already reached zero, and the stage will be increased to the next one. Therefore, it is only happening to saplings placed in the same session. I'm unsure what causes this behavior precisely, but I am in a void world at (0, 0, 0). The specific world seed is 6005780051029461326.