foreach,having wp_query inside, breaks after showing one result

I’m not sure why you are only getting 1 post returned. Your query args look correct, even if as I mentioned in the comment you should be using WP_Query. Therefore, if I had to guess, I would say your looped html markup is off? And is maybe not printing the results correctly. I’ve modified your code to only have 1 get_categories() and categories loop. I don’t know if it is more or less “readable” this way, but it seems to work for me. 

$args = array(
    'orderby' => 'name',
    'parent' => 13
);
$categories = get_categories( $args );

$tabs="";
$panes="";

if( $categories ){

    $tabs .= '<ul id="download-tab" class="nav nav-tabs resp-tabs-list">';

    $panes .= '<div class="tab-content resp-tabs-container" id="download-content">';

    $i=0;

    foreach ( $categories as $category ) {
        $i++;
        $tabs .= '<li><a id="#'.$category->slug.'" href="https://wordpress.stackexchange.com/questions/158615/dtab".$i.'">' . $category->name . '</a></li>';

        $post_args = array( 'category_name' => $category->slug, 
                            'posts_per_page' => -1, 
                            'order' => 'desc' ); 

        $category_posts = new WP_Query( $post_args );

        $panes .= '<div class="tab-pane" id="https://wordpress.stackexchange.com/questions/158615/dtab" .$i . '">';

        if ( $category_posts->have_posts() ) :

            $panes .= '<table class="table table-hover">
                        <tr>
                            <th>DATE</th>
                            <th>TITLE</th>
                            <th>SIZE</th>
                            <th></th>
                        </tr>';

            while ( $category_posts->have_posts() ) : $category_posts->the_post(); 

                $file_size = function_exists( 'rwmb_meta' ) ? rwmb_meta('file_size') : '';
                $file_url = function_exists( 'rwmb_meta' ) ? rwmb_meta( 'file_url') : '';

                $panes .= '<tr>
                            <td>'. get_the_time('d/m/Y') .'</td>
                            <td>'. get_the_title() .'</td>
                            <td>'. $file_size .'</td>
                            <td><a href="'. $file_url .'" class="button3 download_btn" target="_blank">DOWNLOAD</a></td>
                        </tr>';
            endwhile;

            $panes .= '</table>';

        else:
            $panes .= '<p>' . __( 'Sorry, no content found.' ) . '</p>';
        endif;

        $panes .= '</div><!--#dtab' .$i . '-->';

    }

    $panes .= '</div><!--#download-content-->';

    $tabs .= '</ul><!--#download-tab-->';

}

// print it all out
echo $tabs;
echo $panes;

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