Indestructible Rock *updated*

I know this has been reported many times, but it finally happened to me. The dupes just couldn't complete the task of digging even though there was nothing there.

As you can see its at the right where I can't complete the ladder.

EDIT: Okay, when I reloaded the save the problem disappeared, but it did happen in another spot 8 tiles to the right. I was able to make the material inside that tile disappear by having material dug up drop onto it, but the game still registered it was there making it invisible. I'm guessing the problem has to to with the that specific type of tile and whats surrounding it. The Dirt tile with an Algae tile on its right side.