QML iPad app window stays black when launched on external display

Description
Any Qt app using QQmlApplicationEngine (tested with Qt 6.9.1) that is built for iPadOS 18.5 using Xcode 16.4 crashes when launched on the external display.

Note: This is a follow-up issue of the crash QTBUG-106701, which has been resolved, but this black window issue remains.

How to reproduce
1. Build the attached project "iPad-ExtDisplay-Black" on a Mac with Xcode 16.4 installed, and deploy it to an iPad Pro M1 or iPad Air M1 with iPadOS 18.5 or later.
2. Connect an external display to the iPad.
3. Connect a Bluetooth mouse to the iPad, so that you can navigate on both screens.
4. Go to Settings, Display & Brightness, Arrangement, and disable "Mirror Display" so that you get an extended desktop on the external display.
5. Using the mouse, launch the app on the external display. To do this, click the Launcher (list of all apps) icon in the dock on the external display, then click the app's icon.

Result: Window stays black.

Workaround: Launch the app on the iPad's screen, then tap the top center "..." button and tap "Move to Display" (but then it's too small, which is QTBUG-106709).