AVAudioPlayerNode: File not played if scheduled after two or more buffers

Using AVAudioPlayerNode, you can schedule multiple buffers and files, and they will (or at least should) play in sequence. However, I'm finding that some sequences don't work. If you schedule a file after two or more buffers, the file doesn't play. Other combinations work. For example, scheduling a file after only one buffer works, as does scheduling buffers after files, and so on.

Here's a self-contained example (except for the audio file) that demonstrates the behavior (this can be pasted over the contents of the 'ViewController.swift' file in the Xcode iOS 'App' template):

Code Block 
import AVFoundation
import UIKit
class ViewController: UIViewController {
    private let engine = AVAudioEngine()
    private let player = AVAudioPlayerNode()
    private var file: AVAudioFile! = nil
    private var buffer: AVAudioPCMBuffer! = nil
    override func viewDidLoad() {
        super.viewDidLoad()
        let url = Bundle.main.resourceURL!.appendingPathComponent("audio.m4a")
        
        file = try! AVAudioFile(forReading: url)
        buffer = AVAudioPCMBuffer(
            pcmFormat: file.processingFormat,
            frameCapacity: AVAudioFrameCount(file.length)
        )
        
        try! file.read(into: buffer)
        engine.attach(player)
        engine.connect(
            player, to: engine.mainMixerNode, format: file.processingFormat
        )
        
        try! engine.start()
        
        player.play()
        player.scheduleBuffer(buffer)
        player.scheduleBuffer(buffer)
        player.scheduleFile(file, at: nil)
    }
}

It seems like this should play the sound three times, but it only plays it twice.

Is this a bug? Or is there something about the API I'm not understanding?