Steps to reproduce:

1. Visit https://ampbyexample.com/components/amp-next-page/ on a large desktop
2. Scroll down until the next article is loaded and the URL changes to https://ampbyexample.com/samples_templates/news_article/preview/
3. Scroll back to the top.

=> the URL does not switch back to https://ampbyexample.com/components/amp-next-page/

Note: this is only the case if the viewport is large enough to contain the original article and the next one. See this screenshot: