I have a similar issue when syncing some 200GB of music with my Galaxy S21 Ultra. Worst of all, it regularly terminates the sync (at 15-30% sometimes 50%) and then re-detects the phone only to start syncing everything again. Same as you, it shows it never synced. Lots of hours lost syncing without success.
Then I tested syncing with only a few playlists. Everything came through fine, didn't fail or anything, but the next time I connected the phone, it started syncing the same songs even though there were no changes made to the playlist. Not ideal if you want to sync your entire music library.
I hate to say it, but I really wish it worked like iTunes where it would detect changes and sync only what's needed, like tags or album art, update the play count, etc. Most of all, that it doesn't fail while syncing. JRiver literally needs to copy files from one drive to another. If File Explorer can do it without failing, surely this program can too. Fingers crossed that syncing improves in the future updates, but as it stands I'm not ready to lose my 200GB music from the phone just yet.