This is infuriating to me. I manually manage my music library and have for years. I buy the iPhone with the most storage so I can keep my entire library with me locally. This used to work great, but has degraded over the last decade. Now when I drag new music to my phone in iTunes nothing happens for minutes, and then if I get lucky it finally starts transferring, but some times nothing happens at all and I have to retry.
Recently when I load new music onto my phone I find that random unrelated album art has been mangled or switched with other albums from other artists. And some music, which exists on my phone's hard drive, is now greyed out and when clicked says "This item is not currently available in your country or region." I am considering switching back to a iPod with an upgraded drive and giving up on keeping music on my phone completely.