Odoo14, External API: custom model works when accessed through python, but not when accessed through PHP

Question

This is a very weird problem I've been grappling with.

I created a custom model, which I need to access through an external API.

The model: (I believe the function itself doesn't matter, it might as well just return "hi"

class StockApi(models.Model):
    _name = "stock.api"

    @api.model
    def get_lot_info(self, vals):
        """
        Returns the best before date of the current batch of a product specified by its SKU
        """
        val = vals[0]
        if val[0].lower() != "sku":
            return "sku is needed"

        product = self.env['product.product'].search([['default_code','=',val[2]]])
        if len(product) == 0:
            return "product not found"
        product = product[0]

        lot = self.env['stock.quant'].search(['&',('product_id', '=', product['id']),('on_hand', '=', True)], order='removal_date asc')
        if len(lot) == 0:
            return False
        
        return lot[0]['removal_date']

This works when accessed through the python api like so, I receive the removal date

common = xmlrpc.client.ServerProxy('{}/xmlrpc/2/common'.format(url))
uid = common.authenticate(db, username, password, {})
models = xmlrpc.client.ServerProxy('{}/xmlrpc/2/object'.format(url))

def odoo(*args):
    return models.execute_kw(db, uid, password, *args)
odoo(
    'stock.api',
    'get_lot_info',
    [[[
        'sku', '=', '1012', 
    ]]]
    )

However, it doesn't work when accessed with the ripcord library through PHP:

$common = ripcord::client($this->url.'/xmlrpc/2/common');
$this->uid = $common->authenticate($this->db, $this->username, $this->password, array());
$this->models = ripcord::client($this->url . "/xmlrpc/2/object");   
echo var_export( $this->models->execute_kw(
    $this->db, $this->uid, $this->password, 
    'stock.api', 'get_lot_info', array(array(array('sku', '=', '1012')))  
    ), true);

I just get this error: array( 'faultCode' => 2, 'faultString' => 'Object stock.api doesn\'t exist',)

The access variables (username, db and so on) are always the same. The PHP API does work with other models. I could successfully pull sale.order, res.partner and so on, which makes this so hard to understand for me.

Edit: out of interest, I put the new model on the production server, and there I can access it with PHP as well. This is somewhat annoying, because it'd be great if I could work on the staging server with this... The staging server is my local PC. Maybe the problem has something to do with this fact.