Hi @Mswg ,
Based on my research and test, the issue would occur when you have Word as the email editor and configured it to show Field Codes, and we could disable this via press Alt + F9 while composing a message, this keyboard shortcut could work for Outlook and Word to toggle between Fields Codes and their values, please kindly check if it could also work for you.
If that's invalid, considering any add-ins may also affect it, in order to avoid the interference of add-ins, please try to restart your Outlook in safe mode to check if the issue has any difference.(Windows+R>type "outlook.exe /safe">enter)
If the issue still exists, in order to perform more targeted test on my environment and further analyze your issue, please provide me with more information about your issue(such as client version and others).
Hope your issue would be resolved soon.
If the response is helpful, please click "Accept Answer" and upvote it.
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.