iTunes Removing Songs From Playlists When Synching... Again...

I had this problem two years ago, complained to Apple, and it seemed to have stopped. After updating to the latest iOS and iTunes, it started again yesterday. I purchased two songs by an artist, added them to a playlist which already contained two other songs by the same artist, and after I synched all four songs were removed from the playlist!! This is really a frustrating and time-intensive problem to manage, and Apple is fully aware of this. The only way to know if songs are being removed from your playlists is to right-click each song in your entire library to see if it was removed, and then re-add them back to their respective playlists. The least Apple could do is provide a user-friendly way to detect playlist song removals.