We're experiencing the exact same problem.

We're using QB Desktop Premier 2019 R15P. Only one user (of 4) is experiencing this. And it's only occurring when using email relay through Outlook. When using a Gmail account, the problem does not occur, and the user is able to edit the template before sending the email and the edits are retained in the actual email that is sent.

The user has Outlook 2019 (all latest updates installed), on Windows 10 (2004).