This seems like intended behavior. You back button is not broken. Clicking back brings you back to the previous page... that page just happens to seem to be part of the current page.
For you, it would seem best to remove the previous page content when adding the new content. That way pressing "back" will be super fast, page loading will be fast, and the web will still be happy.
Well then that's bad intended behaviour. The back button should reverse the loading of a new page after clicking a link. It should not interfere with scrolling at all.
For you, it would seem best to remove the previous page content when adding the new content. That way pressing "back" will be super fast, page loading will be fast, and the web will still be happy.