magento2 – Magento 2 Adding extension attributes to MagentoSalesApiDataShipmentTrackInterface

I’m trying to add a custom track url to the trackings, using extension attributes, i added these in extension_attributes.xml

  <extension_attributes for="MagentoSalesApiDataShipmentTrackInterface">
    <attribute code="track_url" type="string"/>
    <extension_attributes for="MagentoSalesApiDataShipmentTrackCreationInterface">
        <attribute code="track_url" type="string"/>

Cleared the generated codes and caches, interfaces are populating correctly, I used MagentoSalesApiShipOrderInterface::execute and the $tracks argument has data looking like this:

    "track_number": "1Z37X3X41217934386",
    "title": "ups",
    "carrier_code": "ups",
    "extension_attributes": {
      "track_url": "blah"

So everything looks fine and the extension attributes data are passed fine down to MagentoFrameworkModelAbstractExtensibleModel, however in MagentoFrameworkApiAbstractSimpleObject which is what MagentoSalesApiDataShipmentTrackExtension extends from, $data becomes an empty array, i still can’t figure out why this is happening, i’m in dev mode and shouldn’t need to run DI or something like that, any ideas?