API issue returning object

Question

We have this API where we save different fields, but the object $lines we have created doesn't save properly. The code below are the getters and setters for the object $lines.

Save method API:

public function save(\Vendor\SizeCurves\Api\Data\SizeCurvesInterface $sizeCurves)
    {
        if (
            ($object = $this->get($sizeCurves->getExternalKey()))
        ) {
            $sizeCurves = $this->updateData($object, $sizeCurves);
        }

        try {
            $this->sizeCurvesResource->save($sizeCurves);
        } catch (ValidatorException $e) {
            throw new CouldNotSaveException(__($e->getMessage()));
        } catch (\Exception $e) {
            throw new CouldNotSaveException(__('Unable to save size curve %1', $sizeCurves->getExternalKey()));
        }

        return $sizeCurves;
    }

Model of SizeCurvesInterface:

    /**
     * @param \Vendor\SizeCurves\Api\Data\SizeCurvesLinesInterface[] $lines
     * @return mixed
     */
    public function setLines(array $lines = null)
    {
        $this->setData('lines', $lines);
        return $this;
    }

    /**
     * @return \Vendor\SizeCurves\Api\Data\SizeCurvesLinesInterface[]
     */
    public function getLines()
    {
        return $this->getData('lines');
    }

Interface:

/**
 * @return \Vendor\SizeCurves\Api\Data\SizeCurvesLinesInterface[]
 */
public function getLines();

/**
 * @param \Vendor\SizeCurves\Api\Data\SizeCurvesLinesInterface[] $lines
 * @return mixed
 */
public function setLines(array $lines = null);

Here you can see the answer I receive with postman, where I don't receive any information about $lines