How to override user_picture per comment?
A dump of the $variables array is probably causing out-of-memory problems as preprocess variables in D8 can have some heavy depth to them.

Anyway, you can retrieve the image url from $user_picture render array which has a reference to the comment's user:

  $comment_user = $variables['user_picture']['#user'];
  if ($user_image = $comment_user-&gt;user_picture-&gt;first()) {
    $user_image_uri = $user_image-&gt;entity-&gt;uri-&gt;value;
  }