only 1 category visible in the portfolio

but if I then open one of the two portfolio entries I can scroll to the other entry

you belong to the post navigation arrows left and right. If those entries should stay only in the same category …
can you try this in your child-theme functions.php:

add_filter('avf_post_nav_settings','enfold_customization_postnav', 10, 1);
function enfold_customization_postnav($settings){
  $settings['skip_output'] = false;
  $settings['same_category'] = true;
  return $settings;
}
add_filter('avf_post_nav_settings','enfold_customization_postnav', 10, 1);

if it does not work . we can try a different way …

first tell me if my suggestion from above is correct.
it belongs to the post-navigation left/right arrows? If you have opened a single post – there are left and right arrows to navigate to the other posts.
And now you only want to reach other posts inside the same category ?

And you put that filter to the child-theme functions.php ?

try : maybe the is_fullwidth setting is only missing

function enfold_customization_postnav($settings){
  $settings['is_fullwidth'] = false;
  $settings['skip_output'] = false;
  $settings['same_category'] = true;
  return $settings;
}
add_filter('avf_post_nav_settings','enfold_customization_postnav', 10, 1);

here is the github part showing the long version with comments on that filter:
https://github.com/KriesiMedia/enfold-library/commit/969ff37a8219fee349bb94fcfdbd28fd02d22713