Isn't it reasonable to expect Next/Previous links from within a category to stay in that category?

I am using a theme to build my company’s new website. In showing it to my supervisor they were extremely displeased when after selecting a category in the portfolio section, and then selecting an item from that category, if they click on next or previous it sends them to items that are not within that same category. When I pointed out this issue to the theme maker they said that a solution would be “only possible with code customizations, that you will need to make by yourself or with 3rd-party help. Next and Previous links use default WordPress functions”.

This is not a WooCommerce plugin, but just to make an analogy - if you were on a site, and you clicked on socks, then clicked on next and instead of showing you another sock it showed you a tshirt, then a shoe, then another shirt, etc wouldn’t you think the site didn’t work very well?

If I’m correct, the theme author use the function the_post_navigation to show prev/next post link. The solution is simply enough since this function already cover the “same term” option - more info

What you need to do is add “in_same_term” and “taxonomy” argument to the function. Not sure how your theme code is, but it should similar with code below:

the_post_navigation( array(
    'prev_text'                  => __( 'prev chapter: %title' ),
    'next_text'                  => __( 'next chapter: %title' ),
    'in_same_term'          => true, // set to true
    'taxonomy'                 => 'category', // define the term, e.g category, post_tag, 
) );

edit: yes, next & previous link is default WP function, and yes in most themes - we author not include these “in_same_term” args into the function. So, in order to make the theme “perfectly” work with your needs - you will need to make some adjustment in here and there (including code customization and use of 3rd-party plugins)

Good luck!