Does ajax used in views call preprocessing hooks?
In template "views-view-fields.html.twig" there is a three variables returned : "view", "fields", "row" you can see this here 
.

So to add your custom variables you need to put it into these three variables like below

 function dt8_preprocess_views_view_field(&amp;$vars)
  {
    $vars['row']-&gt;my_var = 'Hey';
  }


and in our template "views-view-fields.html.twig" we print int like this

{{ row.my_var }}