Next/Prev in shop showing all categories

Don’t know if this is totaly different to ismaels snippet – but can you try:

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

And think of caching and merging etc. to refresh.

I think the problem is not that Ismael’s snippet does not work. I have tested Ismael’s snippet – and it works here on my test installation.
But the problem occurs when you have multiple “product categories” for a product.
See e.g. the shop demo there are “men” and “women” products – but they both have : “casual” as another category.
( here again ismaels snippet)

function avf_post_nav_settings_mod($settings){
  if($settings['type'] == 'product'){
    $settings['taxonomy'] = "product_cat";
    $settings['same_category'] = true;
  }
  return $settings;
}
add_filter( 'avf_post_nav_settings', 'avf_post_nav_settings_mod', 10, 2); 

If now a men’s product is called, which is also sorted with casual, then it can be that also women’s products are displayed.
I removed all product categories from men products. By that you only have men product post navigation.