Same issue. Not sure how this one was missed. Almost 30 years of Outlook usage, and it has always had this feature. Huge oversight by the dev team here that makes the new Outlook completely useless to me. Who opens just one email at a time? I've switched back for now.
What's frustrating here is that this is a clear bug, yet we're asked to submit "feedback" that it will "hopefully... be included in a future update". No... this is a bug. It needs to be reported as a bug by a Microsoft employee. We, as the users, should not have to beg via feedback portal for a bug to be fixed.