Thanks for your reply. I'm running version 3.4.7805, and I would have downloaded the installer from the MusicBee site.
In the error log, a few errors like this: "Error HRESULT E_FAIL has been returned from a call to a COM component." These are from specific files that failed to sync during one attempt. But this is an error I've seen occasionally before, usually fixed by just retrying the sync. No errors from when the sync "completed" without actually completing.
I was trying to sync about 4,000 files (though most of them were already on the phone, it was displaying as 4,000 when attempting to sync). Using playlists, if that matters. I just tried again syncing a much smaller number of files (13), and that worked. Notably, it deleted everything else as appropriate but didn't prompt me before doing so like it usually does. I definitely still have that option checked in the settings, so maybe that's part of the error too.