I don't see this as a bug so much as how the worker AI prioritizes where to dig.
When hollowing out an area that connects to a dug out path in two places, rather than dig the entire path the whole way through, on the very last panel, the ants all leave the newly dug out space and go around to the other side to dig out the last panel. This takes extra time to complete the job when they could just dig out the last panel form inside the area they just finished with. I selected the last space with the other ones I wanted dug out in a group, so I'm not sure why this last block is being singled out. I've tested this in both "The Nest" and in the Formicarium missions, and the same behavior occurs.
When hollowing out an area that connects to a dug out path in two places, rather than dig the entire path the whole way through, on the very last panel, the ants all leave the newly dug out space and go around to the other side to dig out the last panel. This takes extra time to complete the job when they could just dig out the last panel form inside the area they just finished with. I selected the last space with the other ones I wanted dug out in a group, so I'm not sure why this last block is being singled out. I've tested this in both "The Nest" and in the Formicarium missions, and the same behavior occurs.