I'm having the exact same issue with QuickBooks Pro 2021.  I was always able to edit the default email template before I clicked on send, and it would save the changes.  Now it saves nothing.  I have to go to my Outlook outbox, reopen the email and make the changes there, which is quite time-consuming.  I never had this problem before, so it must be a recent update.  I'm running the most recent update.  I also did the rebuild and verify procedures, and there was no problem found in my data.