Conflict with get_posts and the_content

I have this snippet in my header.php to display a slider:

    <?php
    $args = array(
        'post_type'             => 'post',
        'posts_per_page'        => 6,
        'meta_query'            => array(
            array(
                'key'           => '_thumbnail_id',
                'compare'       => 'EXISTS'
            ),
        )
    );
    $posts = get_posts( $args );

    global $post;
    foreach ( $posts as $post ) {
        setup_postdata( $post );
    ?>
    <div class="immtestata">
        <div>
            <span><a href="<?php echo esc_url( home_url() ); ?>/category/<?php $category = get_the_category(); echo $category[0]->category_nicename; ?>"><?php $category = get_the_category(); echo $category[0]->cat_name; ?></a></span>
             - 
             <a href="<?php the_permalink(); ?>"><?php the_title(); ?></a>
        </div>
        <img style="width:1000px;height:288px" src="<?php $src = wp_get_attachment_image_src(get_post_thumbnail_id($post->ID) , array(1000, 288) , false, ''); echo $src[0]; ?>" />
    </div>
    <?php
    }
        wp_reset_postdata();
    ?>

But it conflicts with my content.php as the_content() no longer displays the actual post content but the latest recent post! I can't figure out why!

No conflict instead with query_posts (but I know it shouldn't be used).

    <?php
    $args = array(
        'post_type'             => 'post',
        'posts_per_page'        => 6,
        'meta_query'            => array(
            array(
                'key'           => '_thumbnail_id',
                'compare'       => 'EXISTS'
            ),
        )
    );
    query_posts( $args );
    while ( have_posts() ) : the_post();
    ?>
    <div class="immtestata">
        <div>
            <span><a href="<?php echo esc_url( home_url() ); ?>/category/<?php $category = get_the_category(); echo $category[0]->category_nicename; ?>"><?php $category = get_the_category(); echo $category[0]->cat_name; ?></a></span>
             - 
             <a href="<?php the_permalink(); ?>"><?php the_title(); ?></a>
        </div>
        <img style="width:1000px;height:288px" src="<?php $src = wp_get_attachment_image_src(get_post_thumbnail_id($post->ID) , array(1000, 288) , false, ''); echo $src[0]; ?>" />
    </div>
    <?php
    endwhile;

    wp_reset_query();
    ?>

Why is this happening with get_posts?