Problem with current-category-ancestor

I’ve just been updating a site at https://www.education-interaction.co.uk. I added a new page for research under case studies and the problem I have is that when I click on that link it goes to an archive of the research case study instead of just directly to the case study itself. In the source code the link reads like this:
<li id=”menu-item-538″ class=”menu-item menu-item-type-taxonomy current-menu-item current-category-ancestor menu-item-538″>Research

It seems to be something about going to current-category-ancestor?
Any ideas how to fix this would be great.
Many thanks!