Custom menus changing themselves on Category/Tag archives

Take a look at: https://www.omnipublic.co.uk/sites/fantasygirluk/

(Don’t worry, though it is an ‘adult orientated’ site there’s no real content or bad language on there yet and its safe for work, one ‘bunny girl’ (clothed) aside).

Now, I’ve set up the top menu as a custom menu within wordpress and I’m calling it with:

<?php wp_nav_menu( array('menu' => 'top_menu' )); ?>

So far so good, except when you click onto of the three links (they all go to category archives) the menu changes and shows a list of pages instead. This only happens on the category and tag taxonomies (not, for instance, the custom tax I’ve got called ‘girls’ or on date archives).

WTF?!