not really a bug I guess, but update of thumbnail missing.
After moving or removing pictures in background on disk (OK, not a smart behaviour), a refresh-(button) might help to show currently existing/non existing pictures again in extra data.
Thumbnails are created in memory when you open a catalog and you want to show pictures the first time. If you manage pictures in AMC, thumbnails will be updated, but if you not manage picture in AMC, it will not see you delete or update the picture on your hard drive so thumbnail will not be updated. You have to reload catalog to show your updates.