Why is my QtWebEngine still trackable with cookies disabled?

Jez1337

Hello everyone, I'd be grateful for any light shed on the following mystery!

The problem
Some websites still know which pages my QtWebEngine browser has visited, even though:

It uses an off the record profile
It has a filter which blocks cookies from being stored in memory

How are these pesky websites still doing it?!

Steps to reproduce

[a] https://nebbia.fitness/en?countryCode=OTHER
[b] https://nebbia.fitness/eu/c/womens-leggings-sweatpants/classic-hero-high-waist-leggings?color=0

In any browser, ensure you are not browsing in private mode, and visit page [a]
Now visit page [b]. Because you visited page [a] first, you will be automatically redirected

In a Firefox etc, clearing cookies stops this redirection from taking place.

However in my QtWebEngine browser, the redirect still happens, even though it's not saving cookies.

This means that the website somehow still knows I've visited [a] first. How?!?

Possible explanations

Cookies are still being saved - This seems unlikely. I've tested this by spitting out a QMessageBox whenever a cookie is saved. With cookies blocked, those message boxes don't appear, suggesting the filter is working as intended.
Websites can track the browser using another method apart from cookies - No idea what this could be. But whatever it is, closing the browser and launching a new one blocks it from happening.

Relevant code

view = std::make_unique<QWebEngineView>(this);
profile = std::make_unique<QWebEngineProfile>();//Creates an off the record profile
page = std::make_unique<QWebEnginePage>(profile.get(), this);

view->setPage(page.get());
connect(view.get(), SIGNAL(loadFinished(bool)), SLOT(onLoadFinished(bool)));
setCentralWidget(view.get());
	
//Disables cookies
view->page()->profile()->cookieStore()->setCookieFilter(
	[](const QWebEngineCookieStore::FilterRequest &request)
{ return false; }
);

view->load(url);

System info

Qt 5.13.2, x64, using the prebuilt binaries from the Qt website
Compiling using Visual Studio 2017, running on Windows 10