Basic pages are being rendered incorrectly do they differ at all to the article or custom content types in any fundamental way? I have a custom theme, all (custom) content types display as expected except basic pages. Is there something fundamentally different to basic pages that explains/ helps me to troubleshoot why the markup is different?

The site has custom theme/child-theme, custom modules, and a heap of contrib modules so I can't really "paste the code", and as it's a development site I can't give access to it.

The problem I'm experiencing seems to be HTML markup errors, some missing closing elements etc., but I've stepped through most of the code already and can't see anything. Is it something specific to basic pages that Drupal does differently?
Is there something fundamentally different to basic pages that explains/ helps me to troubleshoot why the markup is different?


No, content types are completely generic. Drupal core processes them identically, but of course, custom and contrib code can heavily affect how they work.

You're going to need to check your theme, templates and custom modues, and maybe even contrib modules (depending on what you have installed and how you have them configured). The problem is with one or more of them, not with the content type itself.