iTunes deletes changes from my playlists on my iphone when I sync
This issue has been plaguing me for over a year with several versions of iTunes and OS, it started with my iPhone 6 and continues with my iPhone 8. When I sync my iphone to to iTunes on my computer it deletes newer songs that I've purchased on my phone, and reverts all of my playlist changes that I've made on my phone back to what they were when I last synced to iTunes. Meaning that all changes made on my phone since my last sync are lost.
Additionally when I"m in iTunes and looking at my phone content in there, many of my songs in purchased are greyed out. I can't play them and they look like they want to download. I've tried downloading purchased music and it says that all of my music is dowloaded.
I've done multiple searches and haven't found a solution to my problem. please help
iPhone 8, iOS 11.4.1