Just to answer my own question: I did eventually figure out that I needed to build the macOS app with a proper development profile, one of those so-obvious-I-overlooked-it problems. There's more official documentation up here - https://help.apple.com/xcode/mac/current/#/dev295cc0fae, as well a few more gotchas of my own in another forum thread here - https://developer.apple.com/forums/thread/653004.

Since we're building the game with Unity, it simply creates the .app file, which I then compress to a .zip file and send to our internal team via Dropbox. How would they go about un-quarantining the app, or downloading it without it being placed in quarantine?

Hate to revive an old thread, but how exactly were you "doing it wrong"? I'm having the same problem when I try to build to my ATV 4K from Xcode over wifi, & it's really frustrating.

Wow... I've had this problem for years now, but sure enough just turning the wifi on my Mac off & on again fixes it, & I can connect wirelessly to the ATV 4K. 🙃

Just by way of a post-holiday update, I've noticed that now when I press the X button in the top-right of the Game Center UI, nothing seems to happen, and so I can never return from Game Center back to the game. Has there been much progress on a patch or a workaround for this issue?

Actually I'm testing our game again now, & I can't replicate my earlier issue – pressing X in the GC dashboard does indeed close the view. Might have been a false positive, or something else on our end that was affecting it. Please disregard! And glad to hear about the Present fix. I'll keep an eye on the GitHub project for updates... thanks!

I'm getting the same error: if we don't include the com.apple.developer.arcade-operations entitlement, we're rejected as an Invalid Bundle; if I manually include that entitlement, our provisioning profile is flagged as not including that entitlement. I can attempt to edit our app identifier on this page, but there is no Apple Arcade Operations option listed. This only seems to affect our macOS TestFlight uploads; both iOS & tvOS are uploading just fine.