Is the NormalEmail.dotm being changed? If so, I found that setting it Read Only stops any changes.

1. Close Outlook
   
2. Open %appdata%\Microsoft\Templates
   
3. Open NormalEmail.dotm in Word.
   
4. If it shows "Document1" and not "NormalEmail.dotm" as the current file - STOP!
5. It didn't actually open the Template. Something has it open or it's still Read Only.
   
6. Edit, Save and close the template.
   
7. Properties : NormalEmail.dotm -> Read Only : ON

From here on, when you need to edit NormalEmail.dotm: Set Read Only OFF, do your thing, and then Read Only ON.

https://www.makeuseof.com/windows-change-read-only/