While I like having playlist windows restored in iTunes 12.6, their return is somewhat problematic for many of my scripts. As I mentioned in an earlier post, I spent a few years stripping out references to playlist window in many scripts, particularly the ones that need to access a selection of tracks. In many cases, these scripts can only recognize tracks selected in the main window (the browser window) and cannot recognize a selection of tracks in a playlist window.
I'm updating scripts to accommodate playlist window track selection, but this will take some time to roll out.
In the meantime, be aware that some scripts will need you to select tracks in the main iTunes window. Tracks selected in a playlist window will be ignored.