Override HTTP responses from a Chrome extension

Question

I'm making an extension that will be able to modify request responses. I did some research, found this answer https://stackoverflow.com/a/51594799/16661157 and implemented it into my extension.

When debugging this script, it detects the majority of requests but not all, unfortunately the most important for me are not. So my question is: what could be causing this?

manifest.json

{
    "manifest_version": 3,
    "permissions": [
        "tabs"
    ],
    "web_accessible_resources": [{
        "resources": ["script.js"],
        "matches": [ "<all_urls>" ]
      }],
    "action": {
        "default_popup": "index.html"
    },
    "content_scripts": [
        {
            "matches": [ "<all_urls>" ],
            "js": ["jquery-3.6.0.min.js", "contentscript.js"],
            "run_at": "document_start"
        }
    ]
}

contentscript.js

var s = document.createElement('script');
s.src = chrome.runtime.getURL('script.js');
s.onload = function() {
    this.remove();
};
(document.head || document.documentElement).appendChild(s);

script.js

var _open = XMLHttpRequest.prototype.open;
window.XMLHttpRequest.prototype.open = function (method, URL) {
    var _onreadystatechange = this.onreadystatechange,
        _this = this;

    _this.onreadystatechange = function () {
        // catch only completed 'api/search/universal' requests
        try {
            console.log('Caught! :)', method, URL/*, _this.responseText*/);
        } catch (e) {}
        if (_this.readyState === 4 && _this.status === 200 && ~URL.indexOf('api/search/universal')) {
            try {
                //////////////////////////////////////
                // THIS IS ACTIONS FOR YOUR REQUEST //
                //             EXAMPLE:             //
                //////////////////////////////////////
                var data = JSON.parse(_this.responseText); // {"fields": ["a","b"]}

                if (data.fields) {
                    data.fields.push('c','d');
                }

                // rewrite responseText
                Object.defineProperty(_this, 'responseText', {value: JSON.stringify(data)});
                /////////////// END //////////////////
            } catch (e) {}

        }
        // call original callback
        if (_onreadystatechange) _onreadystatechange.apply(this, arguments);
    };

    // detect any onreadystatechange changing
    Object.defineProperty(this, "onreadystatechange", {
        get: function () {
            return _onreadystatechange;
        },
        set: function (value) {
            _onreadystatechange = value;
        }
    });

    return _open.apply(_this, arguments);
};