Minimum reproduce of mercurius issue

example.js

const Fastify = require('fastify');
const mercurius = require('mercurius');
const path = require('path');

const example = Fastify();
const schema = `
    extend type Query {
        test: Boolean!
    }
    extend type Mutation {
        addTestEvent(userId: Int!): Int!
    }
    type TestEvent {
        userId: Int!
    }
    extend type Subscription {
        testEvent: TestEvent
    }
`;
const resolvers = {
  Query: {
    test: () => true,
  },
  Mutation: {
    addTestEvent: async (_, { userId }, { pubsub }) => {
      await pubsub.publish({
        topic: `testEvent-${userId}`,
        payload: { testEvent: { userId } },
      });

      return userId;
    },
  },
  Subscription: {
    testEvent: {
      subscribe: async (_, args, { pubsub, userId }) => {
        console.log('subscribe event, user is ' + userId);
        return await pubsub.subscribe(`testEvent-${userId}`);
      },
    },
  },
};

example.register(mercurius, {
  schema,
  resolvers,
  path: '/',
  graphiql: 'playground',
  subscription: {
    onConnect: ({ payload: { userId } }) => {
      console.log('Sub connect for user: ' + userId);
      if (userId > 2) {
        return null;
      }

      return {
        userId,
      };
    },
  },
  federationMetadata: true,
});

example.listen(1026).then(() => {
  const gateway = Fastify();


  gateway.register(mercurius, {
    gateway: {
      services: [
        {
          wsUrl: 'ws://localhost:1026/',
          name: 'example',
          url: 'http://localhost:1026/',
        },
      ],
    },
    path: '/',
    graphiql: true,
    subscription: true,
  });

  gateway.listen(1027);
});