Thanks for posting to the Apple Support Communities knotmefromhi,
We understand you're having some trouble syncing your iPhone, when you're able to sync your iPad without issue.
So members of the community can provide the most helpful and relevant responses, we recommend providing some additional information.
For example, how are you syncing music? Right from the computer using a USB cable, or Wi-Fi? Or does music from your computer get uploaded to iCloud, and sync to your iPad automatically?
Are both your iPad, and iPhone set up to sync the same way? That is, if the iPad has "Sync Library" enabled or disabled for iCloud syncing, does the setting for your iPhone match?
What version of iTunes is installed on the computer? What version of iOS is installed on the iPhone XR?
Additional information about your syncing options is available in the resources below:
•Use iTunes to sync your iPhone, iPad, or iPod with your computer
•Turn on Sync Library with Apple Music
Cheers!