Packaged macOS game hang on launch

I recently packaged my game for the first time in a while, had some errors and other cook issues, but eventually got it to succeed. But when I double-click on the packaged game application, it launches and then does nothing. No window is loaded, no level is loaded, nothing. Just an empty application that sits in my dock and in my app switcher, but no windows or game to be seen. I attached Xcode to the running application process and it shows all threads are waiting on a semaphore or signal (I’m not sure how reliable that is on a packaged build, though). I sent this packaged build to my buddy on another Mac, and the same thing is happening. I then tried packing and running on Win64, and it works just fine. Any ideas what might be happening here or how to diagnose the issue? Thanks!
(EDIT)
Here is some more info:
I opened the game and attached Instruments to it. Here is the thread window for the -[FCocoaGame …] thread:

249649-unknown.png

It looks like this thread is hung on a mach message and is likely what holds the semaphore lock for the other threads. All of the other worker threads for the game are stuck on psynch_cvwait:

The [FCocoaGameThread] stack trace suggests that its getting stuck when setting up the game window. Can anyone provide any more insight in this? Epic? Is this an issue with our fullscreen settings or something?

This appears to be caused by the FullscreenMode setting in my DefaultGameSettings.init file. If FullscreenMode is set to any value other than 2, the game hangs when launching. Otherwise, the game launches into a non-fullscreen window and behaves as normal. This is a MAJOR macOS bug; Epic, please advise.