The Microsoft Ribbon control has a seperate way of handling the ability to tab through the controls contained inside a ribbon group.  Our version of the control, XamRibbon, uses the same functionality.  The Ribbon guidelines documentation discusses how this is done.
http://msdn.microsoft.com/en-us/library/windows/desktop/cc872782.aspx#concepts

In order to tab through controls in the XamRibbon you first need to press the Alt key.  This should cause the control to enter keytip mode.  Once in this mode you can press the Tab key to cycle through the controls.  This will take into account your TabIndex settings.

Hi Eldin,

Thank you for posting your question on the forums.  I'm putting together a sample to test your issue and will have an update for you by Friday.