Setting UINavigationItem rightBarButtonItems when pushing a viewcontroller into a navigation stack

Just like the title says, I'm trying to replace the rightBarButtonItems for a view controller when pushing a new controller using the navigation view controller. Weirdly, sometimes this works just fine and sometimes not at all.

Swift 4.2, XCode 10.3

The code

Push the VC:

 DispatchQueue.main.async {
                guard let vc = UIStoryboard(name: "ExampleStoryboard", bundle: Bundle.main).instantiateViewController(withIdentifier: "ExampleViewController") as? ExampleViewController else {
                    return
                }

                vc.data = Data(JSON: newDataJson)
                vc.delegate = self
                vc.images = self.images
                self.navigationController?.pushViewController(vc, animated: true)
            }

Set the barButtonItems:

override func viewDidLoad() {
        super.viewDidLoad()
//        [Some other stuff here...]
        let btn = UIButton(type: .custom)
        btn.setImage(#imageLiteral(resourceName: "icon-image"), for: .normal)
        btn.frame = CGRect(x: 0, y: 0, width: 30, height: 30)
        btn.imageEdgeInsets = UIEdgeInsets(top: 5, left: 5, bottom: 5, right: 5)
        btn.addTarget(self, action: #selector(performAction), for: .touchUpInside)
        self.navigationItem.rightBarButtonItems = [UIBarButtonItem(customView: btn)]
        }
    }

I've read as many questions regarding this topic on SO and various other websites as well as the developer documentation for navigationItem and UINavigationViewController and anything else I could think of. So far I haven't been able to find anything to solve this.

I've tried putting the bar button code in viewDidLoad, viewWillAppear, viewDidAppear, didMoveTo(parent:)... I also tried putting it in a button in the pushed VC and no dice. But this has worked in this exact situation before on previous builds.

The rootVC has rightBarButtonItems which would be overridden by this. I tried setting the parent items as well.

I've also tried this with/without DispatchQueue.main.

I was able to confirm through debugging that the navigationItem does not match the root navigationItem when this is attempting to be set.

Any further info about what might be happening or how to work around/fix this would be very appreciated.