json_decode on PHP fails to convert an array of JSON objects

I have the following code in Laravel 5.4.* to store an array of JSON objects.

if ( $request->get('extra3') ) {
   $samples = $request->get('samples');
   //return $samples;
   Sample::insert(json_decode($samples, true));
}

I had an error for a missing value that is not nullable, so I started the debugging.

This is the return of the $samples, just before the encoding:

[
  {
    "id": 16,
    "contest_id": "35",
    "product_id": "2",
    "supplier_id": "2",
    "quantity": "5",
    "type": "0",
    "created_at": null,
    "updated_at": null
  },
  {
    "contest_id": 35,
    "type": "0",
    "product_id": "4",
    "supplier_id": "3",
    "quantity": 3
  }
]

The first object was already in the DB and was loaded in an Angular model. The second object was a new added from a form (that's why id and timestamps are missing)

As you can see, the "quantity" is a string on the first (one returned from HTTP request) and an integer on the second.

However, if I commented out the return and let the eloquent runs, the insert is messed up.

SQL: insert into \"samples\" (\"contest_id\", \"created_at\", \"id\", \"product_id\", \"quantity\", \"supplier_id\", \"type\", \"updated_at\") select 35 as \"contest_id\",  as \"created_at\", 16 as \"id\", 2 as \"product_id\", 5 as \"quantity\", 2 as \"supplier_id\", 0 as \"type\",  as \"updated_at\" union all select 35 as \"contest_id\", 4 as \"created_at\", 3 as \"id\", 3 as \"product_id\", 0 as \"quantity\", ? as \"supplier_id\", ? as \"type\", ? as \"updated_at\")

Edit:

This is part of the error I have:

Integrity constraint violation: 19 NOT NULL constraint failed: samples.supplier_id

As you can see, Laravel complains for NULL supplier_id. But as you can see the "supplier_id" is in both objects.

via Tasos