When you hit the Enter key and a number simultaneously the number is removed from the Value of the RadMaskedNumericInput

Workaround:
Handle the KeyDown event of the RadMaskedNumericInput control, when the Enter key is pressed.

private void INPUT_KeyDown(object sender, KeyEventArgs e)
{
    if (e.Key == Key.Enter)
    {
        if (INPUT.Value.ToString().Length > 0)
        {
            RadWindow.Alert(INPUT.Value.ToString());
        }
        e.Handled = true;
    }
}