hook_views_pre_render - how to print field x after each loop
This isn't the answer I was looking for, but it solves the issue until a better method is found. 

Basically, the problem is that a specific field - a custom text 'find out more' field was showing three times (for example) when a row had three authors (entity ref. fields) 

Since the 'find out more' field was the last item in each row, I overrode the "views-view-unformatted" twig template and, inside of the 'for' loop I added an extra bit, to change the class of the last item. Then, with CSS, I could hide the fields with 'views-row' class, and display 'views-row-last'

Here's what I added to that twig file: 

    {% if loop.last %}
        {%
        set row_classes = [
          default_row_class ? 'views-row-last',
        ]
       %}

    {% endif %}