subscription update_manual() not working

I’m trying to toggle a subscription manual/auto renewal, using the WooCommerce Subscriptions plugin, but it doesn’t seem to be working. I have the “Accept Manual Renewals” enabled in the settings and the code does execute as expected. The call is made via ajax as follows:

$subscriptions = wcs_get_subscriptions(['subscriptions_per_page' => -1]);
    foreach((array)$subscriptions as $id => $subscription) {
        if ($desiredSubscriptionID == $id) {
            if ($subscription->is_manual()) { // always returns false
                // switch to auto
                $subscription->update_manual(false);
                $response["type"] = "auto";
            } else {
                // switch to manual
                $subscription->update_manual(true);  // returns true as it should
                $response["type"] = "manual";
            }
        }
    }

$subscription->is_manual() always returns false no matter if I do $subscription->update_manual(true) or $subscription->update_manual(false), but those functions do return correctly true/false as per the documentation of the function – it’s just not saved for some reason.

Any help would be greatly appreciated