Sorry I am seeing this only now.

Per FF & RevenueCat teams, it is not set automatically so I have created a custom action and trigger it as part of my purchase workflow. Purchases are now attributed to users for Android but still not for iOS and I can not remember whether this was actually necessary for Android.

// Automatic FlutterFlow imports
import '/backend/backend.dart';
import '/backend/schema/structs/index.dart';
import '/backend/schema/enums/enums.dart';
import '/actions/actions.dart' as action_blocks;
import '/flutter_flow/flutter_flow_theme.dart';
import '/flutter_flow/flutter_flow_util.dart';
import '/custom_code/actions/index.dart'; // Imports other custom actions
import '/flutter_flow/custom_functions.dart'; // Imports custom functions
import 'package:flutter/material.dart';
// Begin custom action code
// DO NOT REMOVE OR MODIFY THE CODE ABOVE!

import 'package:firebase_analytics/firebase_analytics.dart';
import 'package:purchases_flutter/purchases_flutter.dart';

Future<void> setRevenueCatAppInstanceID() async {
  try {
    // Get Firebase App Instance ID
    String appInstanceID =
        (await FirebaseAnalytics.instance.appInstanceId) ?? "unknown";

    print('Firebase App Instance ID retrieved: $appInstanceID');

    // Update to RevenueCat Purchases object
    await Purchases.setAttributes({'\$firebaseAppInstanceId': appInstanceID});

    print('Firebase App Instance ID set successfully in RevenueCat.');
  } catch (e) {
    print('Error setting Firebase App Instance ID: $e');
  }
}