That’s the unfortunate way it operates. It no longer remains showing both panes. You have to drag in, select the email you want to view which once you tap on it then gets rid of the dual pane.
This then has to be repeated each time.
I honestly feel for you. I have no idea why Apple did not allow the dual pane to remain open all the time.
I had it on my 7 Plus and it worked brilliantly.
Personally, I always hated that pane being eternally open in Landscape view! It still remains visible in iMessage Landscape view and I wish that one would go away, too! Maybe I wasn't the only one after all. Sorry that it doesn't suit you; I felt that way before the change.