Rapidly spawned falling sand will show fine when the player is not riding anything, but if they get onto a pig, minecart, boat or horse the sand will disappear and only flicker occasionally.

Link to video demonstrating the effect:
https://www.youtube.com/watch?v=5ElJebs7QOk

I expected it to either always show, or never show. The former would be preferable for map making reasons as currently another bug prevents us from properly changing the Time: tag (E.G: To have them last a few seconds rather than disappear instantly). Riding an entity seems like a strange thing to alter whether it displays or not.