adding field in slm_activate does not work

Ok, here is the situation I have.
I am trying to add a field into the slm_activate function in order to be able to relate the product_id to the txn_id (which I read somewhere). The current beginning of code of the function is the following

function activation_api_listener() {
        if (isset($_REQUEST['slm_action']) && trim($_REQUEST['slm_action']) == 'slm_activate') {
            //Handle the license activation API query
            global $slm_debug_logger;

            SLM_API_Utility::verify_secret_key(); //Verify the secret key first.

            $slm_debug_logger->log_debug("API - license activation request received.");

            $fields = array();
            $fields['lic_key'] = trim(strip_tags($_REQUEST['license_key']));
            $fields['registered_domain'] = trim(strip_tags($_REQUEST['registered_domain'])); //gethostbyaddr($_SERVER['REMOTE_ADDR']);
            $fields['item_reference'] = trim(strip_tags($_REQUEST['item_reference']));
            //$fields['product_id'] = trim(strip_tags($_REQUEST['product_id']));

            $slm_debug_logger->log_debug("License key: " . $fields['lic_key'] . " Domain: " . $fields['registered_domain']);

            global $wpdb;
            $tbl_name = SLM_TBL_LICENSE_KEYS;
            $reg_table = SLM_TBL_LIC_DOMAIN;
            $key = $fields['lic_key'];

With the code as is, everything is ok, AND I get the registered domains when post is made to activate a key.

BUT, When I uncomment the product_id field line, I lose the registered_domain information in the licence manager.
Nothing is done yet with the product_id field, but is causes a different behaviour.

Since I am quite new to php, I can’t see where and why this is causing this kind of behaviour.

Anyone could help me on this please ?

https://ww.wp.xz.cn/plugins/software-license-manager/