Usual mining can get cancelled at any time - when mining tool is selected, you can select the planned area and delete it.
When constructing buildings heartlings first excavate a pit for foundation. This cannot be cancelled unless you cancel (demolish) the whole building.
However a mining area near construction merges with the pit - it cannot be selected any more and you can’t cancel it.
Steps to reproduce:
- Put a building template on the ground.
- Start building (you should pass the short initial automatic “planning” phase and actually begin the construction)
- Put a mining area next to a planned pit for construction while the latter is being dug up.
Mining and construction are different tasks game-wise. While the start of any construction is actually digging, too, it is automatic and the game treats it differently. So the player expects the two “dig tasks” to be separate entities with different behaviour.
The two areas (mining area and construction pit area) will merge. You can’t select and cancel the mining area, it is now considered part of the construction.