iPhone Music not Syncing with iTunes

[ozone]

I have an iPhone 7 and a MBPro running Mojave. I sync my phone with iTunes using a USB-C/lightning cable. Until last night, everything worked fine.

Today, I added a few songs to my iTunes that I wanted to sync. I noticed a few things immediately.

1. I would always be asked on the phone to unlock it to trust the computer it is connected to. Today, that warning disappeared.
2. Certain playlists have disappeared: one playlist clearly listed on my iTunes and checked off to sync does not sync at all into my iPhone. In fact, the playlist has since disappeared from my iPhone and on the device listing within iTunes. However, it is clearly checked as a list to be synced but nothing happens.
3. Some songs on certain playlists have disappeared. It's as if the playlists rolled back four months to an earlier time. However, this isn't the case across all playlists - some have remained up to date. It seems almost random.

I've tried checking things on and off, searched on the web... I have literally no idea why this is happening all of a sudden.

Any help would be really appreciated.