When the InputBehavior is set to Replace, the RadMaskedTextInput control does not clear selected text when you start typing.
To see this behavior:
1. Go to the demo site for MaskedTextInput
2. Select all text in the control
3. Type any character
The first character is replaced with the character I typed, but the rest of the selected text is still there.
If you set the InputBehavior to Insert, the control clears the selected text as expected.
The RadMaskedNumericInput, RadMaskedCurrencyInput, and the RadMaskedDateTimeInput controls (for either InputBehavior value) all clear the selected text before processing the new character.