After sync: Some items (such as ".....") were not copied to iPhone because they could not be found. Why is this?

Latest iOS, iPhone 15. MacBook Pro, latest Sequoia.

After syncing, I'm getting this report about "Some items" missing from the library.

Turns out there's a very long list of songs stuck in this status. But far as I know, they are on the iPhone and play OK there. They just won't play on the laptop.

When I thought there were just a few of them, I clicked on one and got a message like, "It can't be found, would you like to locate it?" I said yes and searched in the finder (outside of Music), found and opened it. The song immediately began playing. I should note that these files seems to be where they belonged:

Music>iTunes>iTunes Music>(Artist)>(Album)>(Song)

I did a few of these,and synced again. This time the list did not include the ones I resolved. But as I said, there are many, so this is not a practical solution.

So what's the solution? And why did it happen to those songs and not the entire library?

Thanks.

[Re-Titled by Moderator]