Try:
1) Settings -> Smart Folder -> Smart Folder time period -> Only unread
2) Settings -> Smart Folder -> Default sort order -> By date/time
3) Back up out of the app
4) Open in smart folder (I use a widget set to open to Smart Folder)
5) Only unread messages should show
6) Open one of the messages, say #3 of 6 if you've got 6 unread
7) Swipe left or right; nothing happens (you don't even get the animation that happens when you can't swipe any further)
Menu -> Mark unread
9) Back up to Account list
10) Go to the inbox folder where your unread messages are
11) Select the same unread message as in #6 above
12) Swipe left or right is fine
13) Keep swiping until you reach the most recent unread message
14) Swipe again, animation shows an attempt to swipe (not shown in #7)
Hope this helps