7.5.77 - I have a category with a custom field in it. It shows up fine in a channel entry form with the standard variable pair. However, it does not display if used in a relationship field. No errors, and it's not unparsed- the content is just not replaced.

{exp:channel:entries entry_id ="27"}
<h3>{title} - {entry_id}</h3>

    {my_rel}
        related title: {my_rel:title} <br>
    {my_rel:categories}
        {category_name} - {cat_sum}
    {/my_rel:categories}        
        
    {/my_rel}
{/exp:channel:entries}

my_rel is the shortname of the relationshp field. cat_sum is the shortname of the category custom field.