Dear Todd B B;
Hope you are doing great.
Based on the information you have provided; it seems that the issue might be related to the email addresses in red. Are they email addresses that you have recently added to your contacts or are they email addresses that you have been using for a while? Are these email address in the same organization? I tested this on my end, and I got the same as yours, I added many addresses in my new outlook, it seems when open first compose email windows and change the "From" address, when select the email address form the same domain, it opens in the same window, however, when selecting an email address from different organization (With different domain), it always opens the new window.
This seems to the design behavior of the new outlook. Thank you for bringing this to our attention. Please kindly understand that many functions are not designed at present. To get this feature more quickly, I suggest creating an entry as a feature suggestion in feedback collection 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. You can also make suggestions on that website and hope Microsoft will notice this in the future.
I will update here if I get any useful update on this feature. Thank you for your understanding and support.
For other concerns, please do not hesitate to add you post in the Microsoft Community Team.