With the 1.3 core restarted using the saved 1.2 dB, here are screenshots of what happened:
There was a Music Folder in my settings-storage which was “Disabled” from when I was messing around prior to this restart. I noticed a lot of music missing. I therefore Enabled this default folder that 1.3 showed, and 6500 tracks were added.
Currently Roon is rescanning 40,564 files which sounds about right. But the rescan has not preserved my metadata edits from before, or the Date Added from before.