rekordbox 4.2.4 which fixed the Sync issue has just been released.
Please update rekordbox to 4.2.4.
The below issue occurred in rekordbox Ver.4.2.3
Please update rekordbox to this version (Ver.4.2.4).
-Sync Manager did not sync playlists.
When you sync playlists which were not synced in Ver.4.2.3, firstly please untick the unsynced playlists and click the Sync button (the arrow icon).
Then, tick the unsynced playlists again and click the button to sync them.
If you still have any issues, please let us know.
CRM, Pioneer DJ