Android app with Realm Flexible not sync'ing

MongoDB Atlas App Services & Realm Atlas Device Sync
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I made an Android app with the Realm Java SDK (the whole app has to be in Java) and got everything to work locally. Now I’m trying to add Realm Sync functionality. I have enabled sync in build.gradle, I initialize the app with the app id and add a simple query to the initial subscriptions. Here is the application class where I put most of the Realm code:

@HiltAndroidApp
public class EsaApplication extends Application {
    private App realmApp;

    @Override
    public void onCreate() {
        super.onCreate();
        DynamicColors.applyToActivitiesIfAvailable(this);

        Realm.init(this);

        String realmAppID = BuildConfig.REALM_APP_ID;

        realmApp = new App(new AppConfiguration.Builder(realmAppID).build());

        Credentials credentials = Credentials.anonymous();

        realmApp.loginAsync(credentials, it -> {
            if (it.isSuccess()) {
                Log.i("AUTH", "Logged in as: " + it.get().getId());
                SyncConfiguration config = new SyncConfiguration.Builder(Objects.requireNonNull(realmApp.currentUser()))
                        .initialSubscriptions((realm, subscriptions) -> subscriptions.add(Subscription.create("allRecordings", realm.where(Recording.class))))
                        .build();
                Realm.setDefaultConfiguration(config);
            } else {
                Log.e("AUTH", "Failed to log in: " + it.getError().getErrorMessage());
            }
        });
    }
}

After creating new objects in the app, they aren’t synced to Realm. I have enabled Development mode, because I don’t have a JSON schema. The only thing in the Realm App Services logs in the successful authentication.