When you clone the last track (or tracks) in a folder, it disappears.
A search reveals it has been created in the NEXT folder!
So you try to drag and drop it back to the bottom of the correct folder. Not possible.
After much fiddling, you find the only way is to drag it ABOVE the last track in the folder, then drag the last track above the cloned one!
This bug has been in existence since Sonar 4.
When the cosmetics of the Folders was adjusted, it would have been nice if also the underlying code had been addressed.
This has been discussed before. One suggestion was making the last row or two of pixels turn into a drop zone when dragging.