Why does UIActivityViewController call viewWillDisappear() on the presenter?

I am presenting a UIActivityViewController to share an .mp4 video from a URL:

let viewController: UIViewController = ... // the presenting view controller
let url: URL = ... // local file

let activityController = UIActivityViewController(activityItems: [url], applicationActivities: nil)
viewController.present(activityController, animated: false, completion: nil)

When the "save video" option is selected, the video is saved, but the presenting UIViewController disappears (and I can see that .viewWillDisappear() is called on it.)

How do I make the presenting UIViewController not disappear?

Note that all of the other share options that I've tried do not have this problem (messages, airdrop, instagram).

I have tried to set the sourceView and sourceRect, but it does not seem to help.

activityController.popoverPresentationController?.sourceView = viewController.view!
activityController.popoverPresentationController?.sourceRect = viewController.view!.frame

I've looked for errors, but didn't find any:

activityController.completionWithItemsHandler = { (a: UIActivity.ActivityType?, b: Bool, c: [Any]?, d: Error?) in
    if let error = d {
        print(error)
    }
}

Also, all of my UIViewController lifecycle overrides call their super, i.e.:

override func viewWillDisappear(_ animated: Bool) {
    super.viewWillDisappear(animated)
}

This is what it looks like:

It's bringing down my entire view with it!

For what it's worth, viewController is setup by calling a segue that is setup in a storyboard:

class LaunchController : UIViewController {

    var performedSegue = false

    override func viewDidLayoutSubviews() {
        if !performedSegue {
            self.performSegue(withIdentifier: "main", sender: self)
            performedSegue = true
        }
    }
}

from Why does UIActivityViewController call viewWillDisappear() on the presenter?