iPhone playlist drops several playlist songs after two syncs with iTunes

I add (and later add again) songs on a named playlists on the iPhone 13 Pro. When I sync (typically a photo sync) via iTunes from the laptop to the iPhone 13, the added songs are removed (disappear) from the iPhone playlist. This seems to take place during every second sync, and not every sync. What am I doing wrong, i.e., is there a specific procedure that I am not following correctly? Is this an iOS issue? Is there a work around? Senior Apple tech advisors are not familiar with what I try to explain during calls into Apple. Thanks!