Note that I have a 2nd RPi running on my same 1Gb Ethernet LAN running Volumio 2.882 and it completes the same full scan in under 40 minutes without error.
SOLUTION: The problem with both versions of MiniDSP’s Volumio 1.111 and 1.121 is that both of these codebases have a regression bug not present in any recent version of the RPi codebase.
When a scan is initiated it never completed (in my case) because whatever mechanism (mpd) is responsible for reading the file, it attempts to validate integrity. In my specific case, I had half a dozen corrupt FLAC encoded files scattered through my library. When v1.111 and v1.121 encountered these corrupt files the error condition was not handled correctly - there was no indication presented to the user, and the scanning process continued to re-validate the same file forever. This behavior is completely different in all recent RPi releases - library scans completed (for me) because corrupt files were skipped - as they should be.
I have lots of log files but this problem is readily reproducible. Place one corrupt FLAC encoded file into a music library and start the library scan process, It will never complete, and what it does complete will not be persisted over a reboot.
This is a bug that should be fixed (or at least communicated to MiniDSP - so they can properly respond to customer issues).