page.php
--------
//...
$doc_categories = Timber::get_terms('document_category');
$context["doc_categories"] = $doc_categories;
//...

page.twig
---------
{% for doc_category in doc_categories %}
  <h1>{{doc_category}}</h1>
  {% set category_docs = doc_category.get_posts() %} {# wrongly getting 0 posts for some cats#}
  {% for doc in category_docs %}
    {{doc.title}}<br />
  {% endfor %}
{% endfor %}

$doc_categories = get_terms( array(
    'taxonomy' => 'document_category',
    'hide_empty' => false,
) );

foreach($doc_categories as $doc_category) {
    echo($doc_category->slug."<BR>"); 
    $args = array(
	'post_type' => 'document',
	'tax_query' => array(
	    array(
	        'taxonomy' => 'document_category',
		    'field' => 'slug',
		    'terms' => array( $doc_category->slug )
		)
	)
    );
    $docs = get_posts($args);
    print_r($docs); // the output shows docs that are missing in the timber loop
}