What you described is usually a result of broken HTML and/or CSS in the affected pages. I suspect that the broken HTML and/or CSS is coming from the header.
I see that the header is different in the working pages and the broken ones. I don’t mean that they don’t look different but the header ID is different. It looks like you duplicated a header and assigned them to a separate set of pages and those pages you assigned the header to are broken.
To test my suspicion, please edit a broken page and select Blank - No Container | No Header, No Footer Page Template. If the issue is resolved, my suspicion is correct so the next step would be to remove or transfer all the custom Header CSS to a notepad and remove elements one by one while checking if the issue would be resolved each time.
If you’ve done those and no elements are left in the header and the issue persists, we need to check what’s going on with the header so we have access to your site so please provide the following information in a Secure Note.
- WordPress Login URL
- Admin level username and password
You can find the Secure Note button at the bottom of your posts.