A possible workaround is to subscribe to the PreviewKeyDown event of the mask then check if the back key is pressed and the whole text is select and set the value to null in order to erase the text of the mask.

private void maskTextInput_KeyDown(object sender, KeyEventArgs e)
{           
	if (e.Key == Key.Back && this.maskTextInput.SelectionLength == this.maskTextInput.Text.Length)
	{
		this.maskTextInput.Value = null;                 
	}
}