Some empty tiles in fog of war are shown as dirt. You can select them for digging, but workers will not run there to dig them.
This seems to be behaving as intended. When tagged black Fog of War always looks like standard earth tiles however it may not be the case when it is finally revealed. Workers will never move to dig a location if they cannot find a path to it, so they wouldn't head towards this tile if there's no way for them to get there.
Customer support service by UserEcho