Have you tried apps like Google Chrome, Discord, Gmail or Youtube? By default, it closes upon clicking the back button. I think this is by default.
Although, other apps override this, in theory adding the script below should prevent closing.
document.addEventListener("backbutton", onBackKeyDown, false);
function onBackKeyDown() {
// Handle the back button
}
Not sure how the Browser - On back button is written that it doesn't override. You can try filing a bug report. Or, just use the script above that I pasted.