Automatic user-settings upgrade

If I understand correctly, configuration files for user settings used to be automatically migrated when a ClickOnce app was updated.
This seems to be no longer the case for .NET 6.

Solutions on the web talk about using MyApp.Properties.Settings.Default.Upgrade() but this didn't work for me and looking at code in LocalFileSettingsProvider showed why: it was designed only to support a directory structure where each Version's settings were stored in a folder of the Version's name and moving to the parent directory allowed previous Versions to be found.
But since ClickOnce apps are effectively different apps, this can't work.

Where is the code that used to do the automatic settings upgrade?

This is the last piece in the puzzle for me in getting ClickOnce to work fully under .NET 6.