Dupe walks on a tile full of invisible sand

There is absolutely no tile there. It's waiting on delivery of sandstone. He walked up to this spot, which I don't think has ever had a tile on it, natural or unnatural, and decided to just stand there and dig.

When he was done, he jumped down and dug the other two tiles there, then he jumped back up on this invisible empty tile and climbed over it.

He went back to town and got some materials and brought them back.

I increased the priority on the tile and another dupe walked up and dug 510 kilograms of sand out of the tile (now that I think of it there had been falling sand above there), then the original dupe came back and finished.

No save/reload monkey business.