Adding and syncing Music?

[Durahl]

Greetings!

I'm having a problem adding Music and synchronizing Playlists from my W10 PC to my WP10 Lumia 1520 and I was wondering if there might be a solution to this.

What basically happens is this:

1. I removed all Online Music from Groove/OneDrive and got rid of the Playlists using the Settings from the Groove Music App on the WP10 PC.
2. I also removed all Music from the WP10 Lumia 1520.
3. I then created a new Playlist on the PC using only Music that is stored on this PC.
4. I followed that by transferring Music from the PC to the Lumia using the Windows Phone Desktop App.
5. I signed into my Account on the Lumia 1520 and waited for the Playlist to Synchronize which eventually happened.

Problem is:
Half of the Songs in the Playlist have a ( ! ) and when attempted to play give the error "We can't play this one. It's not on this device and it's not in a place we can stream it from."

Fair enough...
Might not have transferred so I check the Songs section and look who's there and playing fine?! The song not supposed to be on the device.
Removing the ( ! ) Songs and manually adding the existing variant of them to the respective Playlist on the Phone will then reproduce the error on the W10 PC as the Song will there then not be available despite being on the System.

What is this crap and how can this be fixed?!