Updating unique field fails the unique constraint

Hi there,
I am having some issues when updating unique fields via the JSON API module and any help will be greatly appreciated. I hope I manage to explain it fine:
I have created some Constraints to make a field unique in a content type and in a couple of taxonomy terms.
When I create a new content or taxonomy via the JSON API Extra (I am using Postman to test) it works fine: I can try to create a new content that tries to set an existing value into the unique field and as expected it prevents me:
Here my field "talk_number" has the constraint to be unique (I have created an alias for it in JSON API Extra from "field_talk_number" to "talk_number")

POST api/node/talk:
(where talk_number is unique)

{
	"data":{
		"type": "talk",
		"attributes": {
			"title": "A NEW Talk created using JSON API Extras ",
			"talk_number": "1960001"
		}
	}
}

and the response:

  "errors": [
        {
            "title": "Unprocessable Entity",
            "status": "422",
            "detail": "field_talk_number: Talk number 1960001 is not unique",
            "source": {
                "pointer": "/data/attributes/field_talk_number"
            }
        }
    ]

However when I am trying to update this content/taxonomy and try to set an existing value to the unique field it fails: it updates disregarding the constraint. If I try the same operation from the drupal form it correctly prevents me from updating it.

PATCH api/node/talk/16f868dc-e7ef-4860-9c18-f1431f1567b7

{
	"data":{
		"type": "talk",
		"id": "16f868dc-e7ef-4860-9c18-f1431f1567b7",
		"attributes": {
			"title": "Updating Talk created using JSON API Extras ",
			"talk_number": "1960001"
		}
	}
}

I have attached a screenshot of the feedback I receive when trying to update from the form.

I have added the constraint to the talk number inside a hook_entity_bundle_field_info_alter() like so:

if (isset($fields['field_talk_number'])) {
      // Use the ID as defined in the annotation of the constraint definition
      $fields['field_talk_number']->addConstraint('UniqueTalkNumber');
    }
  }

Is there anything that I might be missing or might this be an issue with the JSON API modules?
thanks for your help!
Luis