Unexpected behavior in WebKit based browsers

I discovered that my application behaves unexpectedly when running in WebKit-based browsers. After some investigation, I found out that there is some issue when a variable is defined in a Javascript block. For example, the following snippet produces the output:

var a = -1;
console.log(a);

5.562684646268e-309

What is interesting is that I was able to reproduce this issue only for integers from the [-127,-1] interval.

However, when a variable is defined using the Number function call (Math.floor work too, for example) like so:

var a = Number(-1);
console.log(a);

... the output seems to be correct:

-1

... so I assume that there is not a problem with printing the output, but rather memory allocation.

Maybe another example that works correctly:

var a = 0 - 1;

Furthermore, I also discovered, that the issue does not occur from the start, but rather that the engine breaks after some (short) time. Again, I tried to eliminate the possibility that my code corrupts the engine/stack/etc., so I created a minimal reproducible example:

// unittest.qml
import QtQuick
Rectangle {
	width: 100
	height: 100
	Component.onCompleted: {
		for (var i = 0; i < 1000000; i++) {
			var c = -1 === Number(-1)
			var d = -1 === -1
			if (c !== d) {
				var a = -1
				console.log("error", i, a)
				return
			}
		}
		console.log("ok")
	}
}

// main.cpp
// ... includes ...
int main(int argc, char **argv) {
    auto& app = *(new QApplication(argc, argv));
    QQuickView *view = new QQuickView;
    view->setSource(QUrl::fromLocalFile("unittest.qml"));
    view->show();
    return 0;
}

The following is printed to log:

error 268680 5.562684646268e-309

I am using Qt 6.4.2 with Emscripten 3.1.31-2.

I was able to reproduce this in the following device-browser environments:

• iPhone 13 Mini with iOS 16.5: Safari, Chrome, Firefox
• iPad Pro 2020 with iPadOS 16.6: all browsers
• M1 Mac Mini with Ventura 13.4.1: Safari, (but Chrome/Firefox works ok)
• M2 Macbook Pro with Ventura 13.4.1: Safari, (but Chrome/Firefox works ok)