Auto Update isn’t working with custom hook, defaults exchange rate to 1.

I’ve created my plugin to retrieve data from NBP (Polish national bank).
When I’m updating rates manually by using “Update All Rates” button everything is okay:

However, when it auto-updates every 5 minutes or so, it defaults to 1.

here is my plugin code:

add_filter('wmc_get_currency_exchange_rates', 'custom_currency_exchange_rates', 10, 4);

function custom_currency_exchange_rates($currency_data, $original_currency, $other_currency, $instance) {
    $custom_currency = get_currency_data($other_currency);
    $currency_data = [$other_currency => 1/$custom_currency['mid']];
    return $currency_data;
}
// Function to get currency data from NBP API
function get_currency_data($currency_code) {
    $transient_key = 'currency_data_' . $currency_code;
    $api_url = 'http://api.nbp.pl/api/exchangerates/rates/a/' . strtolower($currency_code) . '/';
    $response = wp_remote_get($api_url);

    if (!is_wp_error($response)) {
        $body = wp_remote_retrieve_body($response);
        $data = json_decode($body, true);

        if ($data && isset($data['rates'][0]['mid'])) {
            $currency_data = array(
                'rate' => $data['rates'][0]['no'],
                'mid' => $data['rates'][0]['mid'],
                'effectiveDate' => $data['rates'][0]['effectiveDate']
            );

            // Update the last update time
            update_option('currency_data_last_update_time', current_time('timestamp'));
        }
    }   
    return $currency_data;
}

Am I missing something or did something wrong?