iOS requestTrackingAuthorization callback is not on the main thread

The iOS ATT Alert needs to be shown on startup before an app starts, so the remaining app code needs to be run in the completion handler (eg initialising advert code and starting the main game). However, this crashes because the completion handler callback is not on the main thread (after the alert is actually shown and Allow or Ask App Not to Track is selected).

In iOS (Xamarin C# code):

public override void OnActivated(UIApplication application) // RequestTrackingAuthorization must be called when app is Active
{
    if (!this.shownAlert)
    {
        this.shownAlert=true;
        Debug.WriteLine("1) ThreadId={0}", Environment.CurrentManagedThreadId); // ThreadId=1
        ATTrackingManager.RequestTrackingAuthorization(delegate (ATTrackingManagerAuthorizationStatus trackingManagerAuthorizationStatus)
        {
            Debug.WriteLine("2) ThreadId={0}", Environment.CurrentManagedThreadId); // ThreadId=7
        });
    }
}

Prints:

1) ThreadId=1
2) ThreadId=7

And the similarly in Unity:

Debug.WriteLine("1) ThreadId={0}", Environment.CurrentManagedThreadId); // ThreadId=1
ATTrackingStatusBinding.RequestAuthorizationTracking(delegate (int status)
{
    Debug.WriteLine("2) ThreadId={0}", Environment.CurrentManagedThreadId); // ThreadId=4

});

This seems like a bug to me. So how do all the apps and games out there get this to work? Somehow switch to the main thread in the callback? I don’t see any examples of this.