Unexpected default kerning when using attributedText of UITextField

ViewController.swift

import UIKit

final class ViewController: UIViewController {

    private lazy var textField: UITextField = {
        let view = UITextField()
        view.translatesAutoresizingMaskIntoConstraints = false
        view.delegate = self
        view.borderStyle = .roundedRect
        view.keyboardType = .numberPad
        return view
    }()

    override func viewDidLoad() {
        super.viewDidLoad()

        title = "Attributed UITextField"

        view.backgroundColor = .black
        view.addGestureRecognizer(UITapGestureRecognizer(target: self, action: #selector(backgroundTap)))
        view.addSubview(textField)

        NSLayoutConstraint.activate([
            textField.leadingAnchor.constraint(equalTo: view.leadingAnchor, constant: 8),
            textField.centerYAnchor.constraint(equalTo: view.centerYAnchor),
            textField.trailingAnchor.constraint(equalTo: view.trailingAnchor, constant: -8)
        ])
    }

    @objc func backgroundTap() {
        textField.resignFirstResponder()
    }

}

extension ViewController: UITextFieldDelegate {

    func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
        let cursorLocation = textField.position(from: textField.beginningOfDocument, offset: range.location + string.count)
        guard let text = textField.text, let range = Range(range, in: text) else { return false }
        let newText = text.replacingCharacters(in: range, with: string)

        textField.attributedText = addKerning(to: .init(string: newText))

        if let cursorLocation = cursorLocation {
            textField.selectedTextRange = textField.textRange(from: cursorLocation, to: cursorLocation)
        }

        return false
    }

//    func textFieldShouldReturn(_ textField: UITextField) -> Bool {
//        textField.selectedTextRange = nil
//        return true
//    }
}

extension ViewController {

    func addKerning(to string: NSAttributedString) -> NSAttributedString {
        let groupSize = 3
        let kernSize: CGFloat = 4

        guard string.length > groupSize else {
            return string
        }

        let attributedString = NSMutableAttributedString(attributedString: string)

        for index in stride(from: string.length - 1 - groupSize, through: 0, by: -groupSize) {
            attributedString.addAttribute(.kern, value: kernSize, range: NSRange(location: index, length: 1))
        }

        return attributedString
    }

}