Dolby Digital (ac3) over Airplay 2

I'm making an iOS audio playback app using AVFoundation. I'm trying to create the simplest possible implementation of playing a 5.1 Surround Sound audio file via AirPlay to an Apple TV 4K.

The problem: my 5.1 file is downmixed to stereo. My audioSession.maximumOutputNumberOfChannels always returns 2, even if I connect to Apple TV 4K via Airplay and the Apple TV settings have Dolby Digital 5.1 set to on.

I've tested my 5.1 audio file (6 ch, 48000 Hz, 'ec-3' encoded with the official Dolby Digital Encoder utility here: http://dolbyencodingutility.com/) with optical audio in to my Sonos 5.1 system and the channel mappings work correctly.

Code here:

let audioSession = AVAudioSession.sharedInstance()
      
        do {
            try audioSession.setCategory(AVAudioSession.Category.playback)
          
        }
        catch {
            print("Couldn't set category")
        }
      
        do {
            try audioSession.setActive(true)
        }
        catch {
            print("Couldn't init audio session")
        }

        print(audioSession.currentRoute.outputs.count)

        do {
            try audioSession.setPreferredOutputNumberOfChannels(6)
        }
        catch {
            print("Couldn't set six channels")
            print(audioSession.maximumOutputNumberOfChannels)
            print(audioSession.outputNumberOfChannels)    
        }

        //play audio
        let path = Bundle.main.path(forResource: "art.scnassets/Woodwind_6Short_Dolby", ofType: "mp4")!
        let url = URL(fileURLWithPath: path)
      
        do {
            audioPlayer = try AVAudioPlayer(contentsOf: url)
            print(audioPlayer.format)
        }
        catch{
            print(error)
        }
        audioPlayer.play()