Thank you for your interest in Test Studio and the Testing framework.
The described issue seems to be specific for the controls in the application under test. Therefore, my recommendation will be to explore the properties and fields of the Silverlight.UI.TextBox listed here
and verify if any of these will suit the necessary verification.
In case this is still not helpful, I would like to know if there is any possibility to provide us the application under test or a sample one which replicates the same controls. That way, we can test this on our end and possibly find a possible solution for the scenario.
I hope the provided resources will be useful for you. However, in case of any further questions, please get back to us.