Hello Peter Janura,
Good day! Thank you for posting to Microsoft Community.
Based on your description, I understand that you have a concern about the Avenir Next LT Pro font in the New Outlook. Did further tests and consults on my end as well and I'm afraid I'm getting the same behavior. Looks like some fonts including this are not yet included in the new outlook.
I do understand your need for this, and we will surely raise your concerns with our product team and hopefully this can be included in future updates.
If you heavily rely on this, you may consider toggling out of the new Outlook for Windows preview.
Kindly understand that many functions are not designed at present. I'd suggest creating an entry as a feature suggestion in feedback collection directly through your new outlook (Help-->Feedback) or at Outlook · Community (microsoft.com) and many other users could vote for it. Many features of our current products are designed and upgraded based on customers’ feedback.
I will update here just in case I get any update on this.
Please note that nobody here are the product developers/decision makers and has no control over the product. Everyone, including us as community moderator, can only assist you on the query for providing available solutions and feature's availability status within our scope and capability.
Thank you for your understanding.
Best Regards,
Ashraf.