authenticateHandler events not being received on iOS 18

I work on a team that provides an SDK for another game to handle various tasks like authentication. They are experiencing a case where devices using iOS 17 are failing to authenticate with GameCenter, receiving the message "The requested operation could not be completed because local player has not been authenticated." We imagine this is because they still have some setup to finish regarding GameCenter itself, and we're working with them to take care of that.

However, on iOS 18, their app ends up waiting indefinitely for GameCenter authentication messages that it never receives. That's where we're puzzled. We expect them to have the same outcome regardless of OS version.

We initiate GameCenter authentication by setting an authenticateHandler after some initial application setup. The handler has code to account for UI, errors, and successful authentication.

On iOS 17, it's clear that it's getting called as expected because they receive an indication that the player isn't authenticated. But on iOS 18, it looks like the same handler code on iOS 18 isn't being called at all. Are there differences in how iOS 18 interacts with the authenticationHandler that we somehow aren't accounting for? Or is there potentially something else that we're doing incorrectly that is manifesting only on iOS 18?

Here's a simplified version of our login function code (in Obj-C++). There is no OS-specific code, and the job that owns this function does stay in scope until after authentication is complete.

void beginLogin()
{
    // Snip: Check if the user is already logged in.

    // Snip: Prevent multiple concurrent calls to this function.

    auto authenticateHandler = ^(UIViewController* gcViewController, NSError* error)
    {
        if (gcViewController != nil)
        {
            // Snip: Display the UI
        }
        else if (error != nil)
        {
            // Snip: Handle the error.
        }
        else
        {
            if ([[GKLocalPlayer localPlayer] isAuthenticated])
            {
                // Snip: Handle successful authentication.
            }
            else
            {
                // Snip: Handle other case.
            }
        }
    };
    [[GKLocalPlayer localPlayer] setAuthenticateHandler: authenticateHandler];
}