The issue. Have you faced this: A song is successfully transferred with WALTR 2 to your iPhone, but when you try to add it to an existing playlist, it just won't work? The song appears to be in the playlist for just a moment and then it disappears mysteriously. Adding a transferred with WALTR 2 song to the desired playlist using iTunes won't work as well.
The cause. Most likely, it's related to Apple Music. In other words, if you have a playlist on your iPhone which has songs added from Apple Music, the manually added tracks from iTunes or WALTR 2 will not stay there. That's just how Apple decided to make it :(
The workaround. We suggest a simple workaround: create a separate playlist that consists of manually added music only.
Enjoy your favorite music in the new playlist :)