Sorry to interrupt the bash fest, but my guess what’s happening, and something I’ve seen happen before with and without software updates, is Mail app / mail server confusion between sub folders of a mail account.
For example the default folder for ‘sent’ sub folder changes to ‘sent items’, creating two folders containing sent mail. To compound the problem, certain mail servers auto populate and name the common inbox, sent, drafts, etc. to a variety of variations.
Another example is the ‘junk’ folder for the mail app, can be assigned to any folder. But if you create/rename a default mail server ‘spam’ folder To ‘junk’, it will sometimes recreate a separate ‘spam’ folder later.
This also gets compounded with apps like Outlook and providers like Gmail that can monkey with the sub folder structure and have their own unique naming conventions.
The big crime here is the moronic instistence/habit of mail providers/apps using anything other than the obvious default naming convention of inbox, sent, drafts, junk and trash for the common mail directories.