Regardless of the state a gnat mound was in when you save and close a world, be it stage one of rebuilding or the final stage before being destroyed, when you load the world in, they'll return to their pristine, fully workable state.
Steps to Reproduce
Mine a mound a couple of times, save and quit, then load the game back in.
Mine a mound a couple of times, save and quit, then load the game back in.
A developer has marked this issue as fixed. This means that the issue has been addressed in the current development build and will likely be in the next update.