Odd behavior. There must be an explanation but its beyond me. I'm not a programmer:
1. Woke up a sleeping computer. MB, which had been on, happily exported on exit to an empty network folder with correct Unix paths in the playlists
2. Starting and exiting the program after a couple of minutes did not output anything to empty folders
3. Same failure to a local folder without formatting Unix paths
4. Running MB as an administrator made no difference
This tells me that MB won't overwrite old playlists (not even as an admin) and perhaps needs to be running long enough (hidden process) to fully refresh all the playlists before exporting to an empty folder. So the correct strategy for me would be to: delete old playlists > Run MB for a while > Exit
5. Interesting discovery! If I make a change in Preferences>Library>Playlists (e.g. path) and hit apply, MB quietly overwrites all the old playlists in any (local/network) folder
So what I want can be done by making a change in preferences, apply, reverse the change and apply/save again.
I would think the same behavior is needed for auto-export outside the preferences. That to me would be the difference between auto and manual export. In fact right clicking a playlist and choosing export should only mean manual export. Auto should be silent behind the scenes. Furthermore an option to auto-export will be far more useful combined with specifying when to do it, on start, on exit or every so many hours/days. This last paragraph is just my thoughts. Others may not see it that way and programming this is something else altogether. If it sounds reasonable to some more, maybe I'll post it in the feature request section.
@frankz. Thank you so much for taking the trouble to patiently read my posts and suggest a way forward.