Then I'm (almost) out of ideas, but the two suggestions below are both longshots.
First of all, there has been an issue for a while of erratic (or incomplete) syncing, but usually, the symptom is that songs are not copied from the iTunes Library to the device. But it may simply be that no one else has posted here, even though they experience the same symptoms as you. Check the reply that fellow user turingtest2 gave to someone, in which he brings up the matter of erratic syncing, to see if that helps you: I can transfer a playlist from iTunes to … - Apple Community
My only other idea concerns Smart Playlists:
As far as I'm aware, you can only create Regular Playlists on the phone, whereas in an iTunes Library, it's also possible to create Smart Playlists, which can automatically add and remove songs from the playlist, according to the rules of the list.
A Smart Playlist might explain the behaviour that you're experiencing, but as stated, I don't think it's possible to create them on a phone - unless it's a new feature with iOS 13 or later, which is why I mention all this.
To check: look at the Playlist in your iTunes Library and using the screenshot below as your guide, decide which type of icon is shown next to the Playlists in question.

It should be the Regular Playlist type. If it's a Smart Playlist (and I'll be very surprised if it is), post back for more assistance.