I think I raised this issue in ST3 and it was fixed.
When adding songs to a playlist and the pop-over form with the songs in it is visible, if you inadvertently tap outside the area of the pop-over form, the from closes, losing all the selected songs.
Same is true when adding tracks in the audio section of song info. When adding multiple tracks from the file list.
Really needs for the from to stay visible/on-top until closed down with the add or cancel buttons.