Last time on Falling Sand: Sand (or any similar "collapsing" solid such as Snow or Regolith) can fall into tiles that a blueprint is on, but for some reason get rendered as Vacuum instead. Any debris that was on the tile will remain visible on the tile, though the game does mark the tile with a "buried object" flag. As well, liquids trying to flow into the tile cause weird visual distortions...
The tale of the falling sand continues with Mud and Polluted Mud now:
Will the dastardly bug be fixed? Will Falling Sand and her true love be reunited? Tune in next time!
1) find a tile of a collapsing solid (I'll use Sand as the example)
2) place a blueprint (e.g. a Tile) one tile below the Sand.
3) allow your dupe(s) to dig out the square where the blueprint is, below the Sand.
4) watch the sand magically vanish! observe the tooltip still showing the sand being present!
5) if you save and reload, the sand will load in correctly there, though this can be repeated if there is still two or more tiles high of Sand.