Reproducer of issue with including a MarkDown file using an include shortcode.
The appearance of the code example changes depending on whether you view the directly-rendered file (_includes/included.md) or the same file included via a shortcode. When viewed directly, the example code in the included.md file appears correctly. However, when this file is included in regular_page.md, part of the exampel code seems to be have interprested as a heading.
The include shortcode was published in the Hugo Discourse thread. I've modified this code a bit, but the basic core functionality (use .GetPage to get the content of the included file and then render it via markdownify) is the asme. I've played with the code a bit, using RenderString instead of markdownify. That shows teh same result.