Mail.app Cannot Save SMTP Servers, Server List Out of Sync

This is a long standing bug where there's been just pedantic answers for a very obvious issue. SMTP server settings will not save in App… - Apple Community and SMTP server settings will not save in App… - Apple Community

The SMTP server list in Mail.app simply does not work.

In Settings -> Accounts, clicking on an IMAP account, for Outgoing Mail Server (SMTP), the Account pull down also includes and GMail Accounts you have listed. But when you click on "Edit SMTP Server List..." those Gmail accounts aren't there. I get what Apple is trying to do, but this is Mail.app, like usual, is trying to be too clever for its own good. This is a mismatch and confusing.

More annoying, If you add an SMTP server in that list, it often doesn't show up. Sometimes it will go back to the account settings, with the new account selected in the dropdown, but the new account will disappear from the drop down before your eyes and None will be selected! All logs show authentication successful.

This should be really, really simple, but Mail is failing at this. There's something happening in the background and Mail.app is either buggy or not helping users figure out what's happening.