Dear C Burke-Hill,
Good day!
Thank you for posting to Microsoft Community. We are happy to help you.
Normally the steps are as follow
- In the message, click Reply, Reply All, or Forward.
- If you’re working in the Reading Pane, click Pop Out.
Note: If you aren’t working in the Reading Pane, you don’t need to do this. - In the message window, click Format Text > HTML.
For reference. Change the message format to HTML, Rich Text Format, or plain text (microsoft.com)
If there is still no difference, please double check this situation when switching between the new outlook and the legacy outlook on the top right.
Assuming that this situation persists, based on your outlook version, please check if you face the same situation when using the latest update
Version 2209 (Build 15629.20208)
You will find it from this link Update history for Microsoft 365 Apps (listed by date) - Office release notes | Microsoft Learn
Feel free to let me know how it goes for further assistance.
We look forward to your reply for further assistance. Thanks for your cooperation.
Sincerely.
Mac | Microsoft Community Moderator