Cannot change Visual HTML content

I have noticed that a large majority of posts and pages on my website have changed what I would call the ‘Visual HTML content’ (the block text in the centre of the post/page). Just so I’m clear, this is the box directly above the ‘Secondary HTML content box’ in the admin edit view.

All posts and pages (except the Homepage) on my website https://www.healthnetwork.org.uk have randomly managed to replace all their original ‘Visual HTML content’ with ‘Visual HTML content’ of the post or page listed at the botom of the Category list on the left hand side of each article or case study.

An exmple of this would be in the article catergory of the website, the latest article would be Equalities: More equal, more fair.
Now if you look in the left hand column, you will see the list of the ‘Topics’ which are my articles. As you can see the bottom one is Engaging acute clinicians. By looking at this post and navigating through the rest of the ‘Topics’, all of the titles are correct, and the ‘Topics’ list remains the same as it should, however the ‘Visual HTML content’ (centre column text) has been edited without my permission.

Although this has happend to the published website, the WordPress software still has all of my original text in the admin edit view dashboard>pages or posts and even if I update the posts/pages, nothing changes.

I hope you understand my strange problem and can offer some sort of resolution.

Thanks in advance!