I've been working on organizing my art assets and I had a tokens folder with a monsters folder with several monster category folders underneath it. While organizing my undead monsters folder from within the popup window, I created some subfolders, moved assets around, and then discovered that my undead folder replaced the entire monsters folder! I'm not sure what the exact sequence was that I did to cause the issue to arise, but it happened after I created subfolders and moved assets around in the pop-up window.