Clearing the cache in IOS 11.0.3, preview pane off, seems to work, and back arrow works to take to the inbox. So that's a closed item. In IOS 10.2.1, after clearing cache, same issue, once email opened full screen, no functional back buttons working to go back to the inbox. However, if you hit multiscreen to tile multiple windows, you can see the inbox and get back to it, but awkward. Verified on two IOS 20.1.1 phones. Close, but still an issue there, FYI.