Javascript injection goes wrong - android-webview

Question

In our Android project (download manager) we need to show built-in web browser so we able to catch downloads there with the all data (headers, cookies, post data) so we can handle them properly.
Unfortunately, WebView control we use does not provide any way to access POST data of the requests it makes.
So we use a hacky way to get this data. We inject this javascript code in the each html code the browser loads:
<script language="JavaScript">
HTMLFormElement.prototype._submit = HTMLFormElement.prototype.submit;
HTMLFormElement.prototype.submit = formSubmitMonitor;
window.addEventListener('submit', function(e) {
formSubmitMonitor(e);
}, true);
function formSubmitMonitor(e) {
var frm = e ? e.target : this;
formSubmitMonitor_onsubmit(frm);
frm._submit();
}
function formSubmitMonitor_onsubmit(f) {
var data = "";
for (i = 0; i < f.elements.length; i++) {
var name = f.elements[i].name;
var value = f.elements[i].value;
//var type = f.elements[i].type;
if (name)
{
if (data !== "")
data += '&';
data += encodeURIComponent(name) + '=' + encodeURIComponent(value);
}
}
postDataMonitor.onBeforeSendPostData(
f.attributes['method'] === undefined ? null : f.attributes['method'].nodeValue,
new URL(f.action, document.baseURI).href,
data,
f.attributes['enctype'] === undefined ? null : f.attributes['enctype'].nodeValue);
}
XMLHttpRequest.prototype.origOpen = XMLHttpRequest.prototype.open;
XMLHttpRequest.prototype.open = function(method, url, async, user, password) {
// these will be the key to retrieve the payload
this.recordedMethod = method;
this.recordedUrl = url;
this.origOpen(method, url, async, user, password);
};
XMLHttpRequest.prototype.origSend = XMLHttpRequest.prototype.send;
XMLHttpRequest.prototype.send = function(body) {
if (body)
{
postDataMonitor.onBeforeSendPostData(
this.recordedMethod,
this.recordedUrl,
body,
null);
}
this.origSend(body);
};
const origFetch = window.fetch;
window.fetch = function()
{
postDataMonitor.onBeforeSendPostData(
"POST",
"test",
"TEST",
null);
return origFetch.apply(this, arguments);
}
</script>
Generally, it works fine.
But in Google Mail web interface, it's not working for some unknown reason. E.g. when the user enters his login name and presses Next. I thought it's using Fetch API, so I've added interception for it too. But this did not help. Please note, that we do not need to intercept the user credentials, but we need to be able to intercept all, or nothing. Unfortunately, this is the way the whole system works there...
Addition #1.
I've found another way: don't override shouldInterceptRequest, but override onPageStarted instead and call evaluateJavascript there. That way it works even on Google Mail web site! But why the first method is not working then? We break HTML code somehow?