The theme currently sets the taxonomy for known post types like portfolio and gallery automatically in line number 495 of framework/functions/post-functions.php file. Only for other custom post types, you can set the taxonomy. You can modify (or remove) the line above to specify the taxonomy of your own.

Pls override the function mo_get_post_snippets_list($args) in your child theme. You cannot replace the whole file like a template page in the root theme directory.

Can I get a temp access so that I can take a closer look at your change you have in your child theme to make this work? Thank you.

I added this piece of code in your child theme functions.php –

if (!function_exists('mo_get_taxonomy')) {
    function mo_get_taxonomy($post_type) {

        $taxonomy = 'category';

        if ($post_type == 'portfolio')
            $taxonomy = 'contribution';
        elseif ($post_type == 'gallery_item')
            $taxonomy = 'gallery_category';
        elseif ($post_type == 'post')
            $taxonomy = 'category';

        return $taxonomy;
    }
}

It did not work for some reason. Then I changed the parameter from taxonomy to taxonamy (the earlier versions of the theme had this typo) and things are working due to a backwards compatibility code I had in place to accommodate older versions of the theme. Pls check. I have removed the above code.