Already, I have an update and correction.
Since I was thinking about apps permissions, I wound down from this bug report by reviewing System Preferences, Security & Privacy, Privacy. There I found that Terminal.app did not yet actually have the Full Disk Access that I thought I had granted because it had not yet quit and restarted.
So, the above description is not a hard bug, but a strong suggestion, because "sudo mdutil -X volume-path" works without needing a restart of Terminal.app to grant it a new permission.