What I can suggest you to do in order to avoid that behavior is to just move the first RadMaskedTextInput above the ComboBox in the xaml keeping their Grid.Row and Grid.Column positions the same. This way the focus will go into the MaskedTextInput instead of the ComboBox and the controls will remain arranged the same way.
However I would recommend you to use the AutoCompleteBox instead of the ComboBox in this case. You would be able to modify it in order to have similar appearance to the ComboBox by adding a drop down button - you can check the following example from our online XAML SDK repository which demonstrates the exact approach:
If you also have some other requirements for the AutoCompleteBox, you can share them and we will point you the right direction in order to implement them.
Hope this helps.
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