IPA string is not properly voiced on iOS

Question

I'm trying to use AVSpeechSynthesizer with AVSpeechSynthesisIPANotationAttribute but it doesn't seem to work properly.

Here is the IPA notation for the pronunciation of the word "participation" taken from Wiktionary: pɑɹˌtɪsɪˈpeɪʃən. But AVSpeechSynthesizer does not read it correctly.

When I add a custom pronunciation using Settings -> Accessibility -> Spoken Content -> Pronunciations on my iPhone, it gives me this notation: pəɻ.ˈtɪ.sɪ.ˈpe͡ɪ.ʃən, which the iPhone reads correctly.

Why does "pəɻ.ˈtɪ.sɪ.ˈpe͡ɪ.ʃən" work but a string like "pɑɹˌtɪsɪˈpeɪʃən" that is taken from a dictionary does not?

Here is a sample code you can try:

import UIKit
import AVKit

class ViewController: UIViewController {
    
    let synthesizer = AVSpeechSynthesizer()
    
    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view.
    }
    
    
    @IBAction func buttonPressed(_ sender: Any) {
        let pronunciationKey = NSAttributedString.Key(rawValue: AVSpeechSynthesisIPANotationAttribute)
        
        let attrStr = NSMutableAttributedString(string: "foo",
                                                attributes: [pronunciationKey: "pɑɹˌtɪsɪˈpeɪʃən"])
        let utterance = AVSpeechUtterance(attributedString: attrStr)
        
        synthesizer.speak(utterance)
    }
}