Hello Shane Woodward,
Just to be clear on this, the EmptyContent property acts as a watermark text when the RadMaskedTextBox is empty. If you set it to null, then you would never be able to see it, no matter whether the control is empty or not. You would rather want to use this property to display some prompting text like "Enter value here".
Please also note that the RadMaskedTextBox does not support null values.
I tried to reproduce the behavior that you described but I was not able. Could you please provide more details on your setup (the child window) and when you set the Binding.
All the best,
the Telerik team