Thanks for finding this. It looks like the flags attribute is a non-published property of a Track object, i.e., it's a property accessible to MM's Delphi code but not JS. Tracked as https://www.ventismedia.com/mantis/view.php?id=20193

It looks to me that the return value from this function now includes a "flags" attribute that wasn't there before.
What are the flag values and purposes?

ie. app.playlists.getByIDAsync({mm_pl_id}).then(function(playlist) {{ if (!playlist) runJSCode_callback('Could not find playlist'); else playlist.getTracklist().whenLoaded().then(function (list) {{ runJSCode_callback(list.asJSON); }}); }});