When to set AVAudioSession's preferredInput?

I want the audio session to always use the built-in microphone. However, when using the setPreferredInput() method like in this example

private func enableBuiltInMic() {
    // Get the shared audio session.
    let session = AVAudioSession.sharedInstance()
    
    // Find the built-in microphone input.
    guard let availableInputs = session.availableInputs,
          let builtInMicInput = availableInputs.first(where: { $0.portType == .builtInMic }) else {
        print("The device must have a built-in microphone.")
        return
    }
    
    // Make the built-in microphone input the preferred input.
    do {
        try session.setPreferredInput(builtInMicInput)
    } catch {
        print("Unable to set the built-in mic as the preferred input.")
    }
}

and calling that function once in the initializer, the audio session still switches to the external microphone once one is plugged in. The session's preferredInput is nil again at that point, even if the built-in microphone is still listed in availableInputs.

So,

1. why is the preferredInput suddenly reset?
2. when would be the appropriate time to set the preferredInput again?

Observing the session’s availableInputs did not work and setting the preferredInput again in the routeChangeNotification handler seems a bad choice as it’s already a bit too late then.