RealityKit custom session config fails to load assets

I'm just using the standard template out of the box with custom configuration for my SwiftUI + RealityKit project:

func makeUIView(context: Context) -> ARView {
        
        let arView = ARView(frame: .zero, cameraMode: .ar, automaticallyConfigureSession: false)

        let config = ARWorldTrackingConfiguration()
        config.sceneReconstruction = .meshWithClassification
        config.isAutoFocusEnabled = true
        if type(of: config).supportsFrameSemantics(.sceneDepth) {
            config.frameSemantics = .personSegmentationWithDepth
        }
        arView.environment.sceneUnderstanding.options.insert([.occlusion, .collision, .physics])
        arView.session.run(config)

        // Load the "Box" scene from the "Experience" Reality File
        let boxAnchor = try! Experience.loadBox()
        
        // Add the box anchor to the scene
        arView.scene.anchors.append(boxAnchor)
        
        return arView
        
    }

And in my AppDelegate.swift:

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
        
        guard ARWorldTrackingConfiguration.supportsSceneReconstruction(.meshWithClassification)
                else {
                    fatalError("Scene reconstruction requires a device with a LiDAR Scanner.")
                }

        // Create the SwiftUI view that provides the window contents.
        let contentView = ContentView()

        // Use a UIHostingController as window root view controller.
        let window = UIWindow(frame: UIScreen.main.bounds)
        window.rootViewController = UIHostingController(rootView: contentView)
        self.window = window
        window.makeKeyAndVisible()
        return true
    }

Where the only change in the AppDelegate was the guard. The app loads fine, no crash, but the asset/"Box" does not load. Even if I enable debug mode with the config with meshes being displayed, nothing appears. When I don't run arView.session.run(config), the asset loads perfectly fine. What am I doing wrong to crash this?