How to synchronize one to many relationship between Amplify DataStore and DynamoDB?

I am working on a Grocery List App with 2 models: Product and Grocery List. A Grocery List has many Products and a Product belongs to one Grocery List.

# schema.graphql
type Product @model 
{
  id: ID!
  groceryList: GroceryList @connection(name: "GroceryListProducts")
  name: String!
}

type GroceryList @model {
  id: ID!
  name: String!
  products: [Product] @connection(name: "GroceryListProducts")
}

On DynamoDB, as far as I understand, the one-to-many relationship can be queried by using a global secondary index and won't be directly showing on the DynamoDb records:

DynamoDB Grocery List table:

╔═══════╦═════════╗
║ Id    ║    Name ║
╠═══════╬═════════╣
║ L1    ║ List1   ║ 
║ L2    ║ List2   ║
║ L3    ║ List3   ║ 
╚═══════╩═════════╝

DynamoDB Product List table:

╔═════════╦═══════╦════════╗
║ Id      ║ Name  ║ PID*   ║
╠═════════╬═══════╬════════╣
║ O1      ║ Orange║ L1     ║
║ O2      ║ Apple ║ L1     ║
║ O3      ║ Bacon ║ L2     ║
╚═════════╩═══════╩════════╝

*ProductGroceryListId

When I am adding a product and saving it on the DataStore as below:

      const productSaved = await DataStore.save(
        new Product(product)
      )
      await DataStore.save(
        GroceryList.copyOf(currentList, updated => {
          updated.products = [...updated.products, productSaved]
    }))

The GroceryList instance on the datastore will be correctly showing the products children:

// GroceryList item showing on the browser DevTools, tab Application, IndexDB, amplify DataStore:
{
 id: "L1"
 name: "List1"
 products: [
  {id: "01", groceryList: {…}, name: "Orange"},
  {id: "02", groceryList: {…}, name: "Apple"}
 ]
}

However, if I add the sync with the Cloud:

 const subscription = DataStore.observe(Product).subscribe(msg => {
        console.log(msg.model, msg.opType, msg.element);
})

The children will disappear from the GroceryList DataStore instance. How can I prevent it from happenning?

I would like to make sure I can use my app both online and offline.

Thank you in advance for your help!