When turning off Control Tab navigation on an items control that contained some buttons due to wanting to use RB and LB for something outside of noesis, we came across an issue whereby when these buttons were pressed, if the element that was focused had the ControlTabNavigation attached property set to None then it would completely unfocus all elements.
The same thing happened with the Tab key and when TabNavigaition attached property was set to None.
Guessing the function to predict the focus in these cases didn't have a null check before setting the focus to nothing when it couldn't find a suitable element.