If I look at https://en.m.wikipedia.org/wiki/Secretary-General_of_the_United_Nations (this version) on a browser on a mobile phone, the last section I see is "Lifespan timeline". If I tap it, I also see subsequent sections: "Living former secretary-general", "See also", etc., but they are expanded (I don't have to tap them to expand them), and there are no gray horizontal lines under the headings.
When I look at the same pages on a laptop web browser, I see a similar problem: all the sections are expanded, as they are usually on MobileFrontend on laptops, but there are no horizontal lines under the headings.
Note on the screenshot: gray line under "Lifespan timeline", but no gray line under "Living former secretary-general".
I thought that the wikitext is perhaps invalid, but I couldn't see any obvious mistakes when I took a quick look.