Take a look at cleanImagueURL in the for developers section #30784

I saw it. But the problem looks like I can't decode json. I'm only able to echo the subform with media custom field, resulting in a ul li render but no raw data.
Json decode does not work. When i'll be able to get the raw data for sure I will use cleanImageURL, but it seems to be a problem before that.

Where do you get your $myFields from? It looks like that array is built from the ->value attributes of the fields and not ->rawvalue. The former contains a pre-rendered version of the data, the later contains the raw data.

I'm getting custom fields in this way:

// GET CUSTOM FIELDS
use \Joomla\Component\Fields\Administrator\Helper\FieldsHelper;

$jcfields = FieldsHelper::getFields('com_content.article', $this->item, true);
$myFields = array();
foreach ($jcfields as $field) {
	$myFields[$field->name] = $field->value;
}

@giudansky As Hannes said, you have to use rawvalue:

$myFields[$field->name] = $field->rawvalue;

As said, you are getting the data from the wrong place. So I'm closing this here. If you feel like this has been closed wrongly, please respond again and we'll open it again.