When I was new and before I had read forum posts/notes about this, that is exactly how I would leave rocks because I was paranoid that if I finished mining the rock, it wouldn't grow back.
I had some new players do this to my yard a few times. I politely told them that it's better to mine them all the way down, and he apologized. Some people just don't know :)
When I was new, I had expected the rock to grow back but I never had enough energy to mine an entire rock. And, getting a few tens-of-currants for rocks was a lot of riches, I didn't need whole rocks! Ahh, so long ago...
actually, I now wonder why not mine what they want from one or two rocks, rather than scamper about slightly mining them all?? I guess it really just is that they don't want to kill your rocks.
I finally put up a note pole with a friendly tip for new miners that the rock won't grow back until it is completely used up & haven't had a single problem since. and not to brag, but I think mine was worse www.glitch.com/snaps/PUVFEH...