I think I'm having a similar problem. I set up my new 15 Pro Max this weekend. I have thousands of songs on my iPhone. When I set up the new phone and it copied stuff from the old, the process was over quickly, and I don't think it's possible that it could have copied all of the music from the old phone to the new. And, some albums are missing bunches of songs. Also, I changed the setting to stop down-sampling music to 256 ACC, and when I synced the phone again, it did not re-download the songs in their full resolution format.
(Both the phone and computer are running the current iOS/OS.)
(Also, when I connect the phone to my computer, it keeps swtiching to back up my phone to the cloud. I don't want my stuff on the cloud, and check the box to backup to my computer, but it keeps changing back)
Any ideas?