Looks like we can't reproduce the issue or there's a problem in the test case provided.
Sencha - Support Team
Navigationbar - 'back' ui not accepting button text
Text in button is missing. If I change 'navigationbar' to 'toolbar' - text appears.
xtype : 'navigationbar',
ui: 'back', text: 'Back'
Thank you for the report.
Sencha - Sencha Touch Dev Team
The navigationbar component has been renamed to Ext.TitleBar. Ext.navigation.Bar is now a private class used by Ext.navigation.View.