Hello.

I see that when viewing a Category, the meta Title is Correct but the meta Description is actually taken from the first ad listed and not the description we have set in admin / Categories field.

I think this was working right in 3.9. I checked also your demo here, just to be sure it is not my issue. And the problem is also appearing in your demo.

See:

<title>For sale  - Osclass Official Demo Page</title>
<meta name="title" content="For sale  - Osclass Official Demo Page" />
<meta name="description" content="Electronics Ireton, Ut aliquam massa in rutrum convallis. Ut id velit vel est fermentum laoreet vel eu purus. Etiam fringilla tellus sed lig..." />

Any ideas how to fix this?

Thanks

It is intentional as most classifieds site does not have category description

It is intentional as most classifieds site does not have category description

Ok, but we do have full categories descriptions.

@dsf
It is how it is, you could use SEO plugins those allows you to customize meta tags. Most of SEO plugins can do this.