Swift NSAttributedString.Key.foregroundColor not working in some older iOS versions

Question

I'm using the below extension while adding multiple colors to UILabel text. Results are showing as expected in iOS 13 and above, While came to know that it causes issues in iOS 12.4 as showing default UILabel text color regardless of any changes.

I'm initializing NSMutableAttributedString in ViewDidLoad() initially with attributes as well. Wondering what causes it not to work in older iOS versions.

extension NSMutableAttributedString {
    
    func setColorForText(textForAttribute: String, withColor color: UIColor) {
        let range: NSRange = self.mutableString.range(of: textForAttribute, options: .caseInsensitive)
        self.addAttribute(NSAttributedString.Key.foregroundColor, value: color, range: range)
    }
    
    func setColorAndFontForText(textForAttribute: String, withColor color: UIColor, withFont font: UIFont) {
        let range: NSRange = self.mutableString.range(of: textForAttribute, options: .caseInsensitive)
        self.addAttribute(NSAttributedString.Key.foregroundColor, value: color, range: range)
        self.addAttribute(NSAttributedString.Key.font, value: font, range: range)
    }
}