Ok, as far as I can see the problem is that the "WatermarkVisualElement"-contentcontrol in the RadWatermarkTextBox template captures the focus on SHIFT-TAB. If you apply an IsTabStop=false to "WatermarkVisualElement" it works. Could you please confirm this and add it to your next build if that's the correct fix?
And while you're changing the RadWatermarkTextBox-template you could fix another bug: Usually one applies the Telerik styles to the native controls as well to get a consistent look. However, if you to this for scrollviewer like this in app.xaml
you'll notice, that all RadWatermarkTextBoxes loose their border which looks completely stupid. I think this is because in contrast to the native ScrollViewer style, the Metro style for a ScrollViewer stretches the viewer in both directions which eventually hides the border element of RWTB which is under/before the ScrollViewer (see the RWTB template).
To fix this issue you could apply a Margin=1 to the ScrollViewer element ("ContentElement") in the RWTB template so that the border element will shine through.
I would be very glad if you could apply both fixes in the next build.