It might not be a new thing, but I found something very, very strange on relclaiming. I didn't actully use the reclaim at the embark screen, I just embarked on half of my old fortress, which didn't show up on the world map for some reason. Once I had embarked, It showed kobolds all around me, friendly towards me. I assumed that these kobolds were on the map when I abandoned, as I had locked myself away from the outside world and were thus unrevealed when I abandoned. The kobolds killed the forgoten beast that killed my last fortress, and then I ordered someone to build a wall to the caverns, to stop anymore annoying things coming in. Being a dorf, he built the wall on the wrong side, so I ordered it deconstructed.
When this happened, everything went black, as if it was undiscovered. I went back to the top, and found that the stairway had been filled it, somehow. I dug a new one, and realised that the entire fortress had been filled in again, as if I had embarked on a new place. All the items that were down there were released when the stone was dug out. The funny thing was, everyone but my miner was filled in, so he had to race against time to dig them out. Also, now all the trees and shrubs growing are called "tree" and "shrub" without any type, and I also can't cut them down. It is very, very strange...but at least I can mine out the same cluster of magnetite twice
*edit*
After some !!science!! I found that, once the site has been improperly reclaimed, it did not revert when I built and deconstructed a wall outside, but if I created then deconstructed a wall in the caverns it will refill everything. Refilling also puts down more mud in the carverns.