The format of UITextField is changed after clearing the text and inputting new text.

I have a UITextField and set its text format(font/color/underline) by UITextField.attributedText. However, when I clear the text than input text again, the format is changed. It seems the behavior of attributedText is reset by system after clearing the text. Is this a bug or can we have some workaround to keep the consistent format after clearing the text? The code snippet is like below:

    let textField = UITextField(frame: CGRectMake(0, 0, 200, 60))
    let fontColor = .red
    let attrText = NSMutableAttributedString(string: "normal text")
    let range = NSRange(location: 0, length: attrText.length)
    attrText.addAttribute(.font, value: UIFont.systemFont(ofSize: 14), range: range)
    attrText.addAttribute(.foregroundColor, value: fontColor, range: range)
    attrText.addAttribute(.underlineColor, value: fontColor, range: range)
    attrText.addAttribute(.underlineStyle, value: NSUnderlineStyle.single.rawValue, range: range)
    textField.attributedText = attrText