Mined area reverted to original state.

I mined an area and was turning it to a garden. 

What I expected to happen was...:
when planting seeds, game was unresponsive so i restarted the world and everything back to normal

What actually happened was...:
game reverted the area to unmined Tuff and other things. destroying what i had planted and deleting every terra form i made within that chunk

Steps to Reproduce:
1.Clear an area of Tuff 
2.  Terraform said chunk
3.  restart world and chunk reverts to original state