It's using whatever library stuffs an extra page in your history, "More stories to check out before you go!" in a blue banner at the top of the page. (iOS chrome)

Firefox Android, it fills the back history in a loop. With a long press on the back button though I can then jump over the looping page. Back to HN.

On iPhone Safari, after clicking the article link, I cannot navigate back to HN using the back button.

I was unable to go back on mobile Firefox. Going back would instantly redirect me back forwards.

Edit: Tested on desktop firefox, and it works fine there.

I’m guessing you’ve ublock origin or similar on those browsers thats mitigating it. It’s broken on safari (iOS).