Opencart 3.0.3.x changes in some controllers has no effect

Question

I had to start learning Opencart (I'm a newbie) as I just got an Opencart webshop with VQMod to redesign it. The previous developer hardcoded some things, and God knows what else happens in the code.
My problem is that changes in some controllers take effect, while some don't. For example:

url: */index.php?route=account/login
catalog/controller/account/login.php:

class ControllerAccountLogin extends Controller {
    private $error = array();

    public function index() {
        $this->load->model('account/customer');
        exit;
...

This has zero impact; it renders without any problem. It doesn't matter whether I exit, insert valid or invalid code. But the same changes in:

url: */index.php?route=account/login
catalog/controller/common/header.php:

class ControllerCommonHeader extends Controller {
    public function index() {
        $this->load->model('setting/extension');
        exit;

or

url: */index.php?route=account/account
catalog/controller/account/account.php

or

url: */index.php?route=common/home
catalog/controller/common/home.php

Do exit the code. The same thing happens with the corresponding models. Changes in models, some take effect, while some don't. It does not matter what I try; changes in some controllers simply have no effect. It's like (to me) the changed controllers/models wouldn't be loaded at all.

I deleted all caches, including VQMod manually and OC on-site, but I don't think it's a caching issue.
What can be the problem, what should I look for?