When you have email in Drafts Folder save while you create new message and they are marked as unread (the same happen if you click unread from menu on Draft) and you get new emails the tray icon change to unread but when click on each email you got and click draft as the last one you don't have tray change and tray stay as unread.

There should be check if icon is in unread state when you click on draft to mark it as read.

If you click on tray icon and click work offline and then online, then the icon is change to correct state.

It happen often when you get new emails while you create new message and you don't send that message but save it for later.