[artlung]I see the behavior in Firefox. With a very tall browser window I didn't see the behavior. But it appears some of the JS enabled content being loaded manages to pull focus to itself and scroll the parent page such that it's at the top or visible. I don't know that I'd call it scroll being affected, I suspect it's focus that gets affected, and scroll is part of that. Like scroll to an anchor, sortof.
[artlung]could be a scroll() call, or an document.location.href update that moves the page. or a page refresh maybe. those are all guesses. 🙂 if you sort it out I'd love to hear how it works. also you might try adding a loading=lazy to the iframes which could mitigate it.