my bigger beef on this one is that if you have automated workers (yeah, I know what you are going to say), they will put a fort on top of a developed resource, even if you have "workers leave improvements" checked.
Quite annoying... and I didn't see this before 3.17
This happens when an automated worker decides to build a fort at the same time another worker is starting to build the appropriate improvement. The pasture/mine/plantation is completed, however the automated worker building the fort continues on his task. Then you end up with that worker completing his fort and erasing the proper improvement that you might have manually selected. I don't know of a way around it other than refusing to automate workers and checking undeveloped resources for workers dumping forts on them.