Looks like we can't reproduce the issue or there's a problem in the test case provided.
Title Bar in NavigationView not truncating after 'back'
Inside a NavigationView if I push a view with a long title:
The title sets correctly with truncation, but then inside that view I push a new view (a details page), and that title sets correctly.
xtype : 'viewWithLongTitle',
title : 'This is a long title that will need to be truncated.'
The problem is that when I click the 'back' button provided by the NavigationView my title is no longer truncated.