In the most recent build as of this post (git-Spigot-90839e6-0ff499c), when sand is quickly pushed up by a block that is quickly retracted, it breaks. As recently as git-Spigot-23e921f-8b69bdd the behavior was normal, so something has changed since then.

Here's a video showing what the issue is: https://www.youtube.com/watch?v=IiNKRL8NHc