Erroneous "unreachable dig"

Here is the history of the tile in question. It's the one with the downward pointing triangle aimed at it.

Originally, it was not dug out, it was a phosphite tile that was designated to be an insulated tile. It's a little hard to tell but the tile to the left of the word "retaining" is where the dupe goes to stand to affect this tile. It's perfectly reachable.

Anyway, a dupe showed up and dropped 400 kg of igneous rock on that tile. At that point I peered at the tile to see if there was an errand associated with this tile. There was none, and I was on the verge of reporting that as a bug, but there was a dig errand on the tile and a dupe was assigned to it. So good job.

Anyway, a dupe showed up and did the dig errand, and then took off, and left me with this red "unreachable dig" status message.

There is no dig here, and it is not unreachable. There is a build errand associated with tile and the dupes are considering doing it.

The bug here is the red status message.

There is nothing weird going on here, e.g. pathing through deep water, etc., it's just a spurious message.