Similar problem for me, I have a playlist with 100 songs and one is not on spotify. I downloaded from youtube to mp3 and pressed download while on the same network, the playlist has the same song count with the downloaded song, but it doesn't show up within the playlist for me to press to play on my phone. I can play on my computer but it does not show up on my iPhone. I would appreciate it if there was a way anyone could help, thank you.
For next I'd suggest booting your Macbook into Safe mode, and trying to sync while being connected to the iPhone's hotspot connection.
To start up in safe mode:
Start or restart your Mac, then immediately press and hold the Shift key. The Apple logo appears on your display. If you don't see the Apple logo, learn what to do.
Release the Shift key when you see the login window. If your startup disk is encrypted with FileVault, you might be asked to log in twice: once to unlock the startup disk, and again to log in to the Finder.