Posts without featured image using other post's featured image

Question

I am using the following code (see bottom) to display a custom post called 'artists'. I am also using <?php if (has_post_thumbnail( $post->ID ) ) so I can set the background of a div using the post thumbnail (I don't think this is the problem).

I want to display all of the posts on one page so use 'numberposts' => -1,. I have noticed that the posts after the first twelve posts are using another posts featured image if they do not have one assigned.

If I assign a featured image to any post in question then it does show the correct image. I just don't understand why every post after the first twelve are using another posts featured image. Is there something wrong with the loop as I'm using 'numberposts' => -1,? There's no harm in this as I'll be assigning an image to each one but want to understand why this might be happening.

   <?php

$args = array( 
    'post_type' => 'artists',
    'numberposts' => -1,
    'orderby' => 'title',
    'order' => 'ASC' 
);

$myposts = get_posts( $args );
foreach ( $myposts as $post ) : setup_postdata( $post ); ?>

<?php if (has_post_thumbnail( $post->ID ) ): ?>
<?php $image = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), 'single-post-thumbnail' ); ?>
<?php endif; ?>

    <li class="artist-list artist-mobile">
        <a href="<?php esc_url( the_permalink() ); ?>">

                <div class="artist-box mobile" style="background-image: url('<?php echo $image[0]; ?>')">
                <div class="mobile-link"></div>
                <div class="info-back">
                <div class="info-post"><p>VIEW ARTIST</p></div>
                <div class="cross">+</div>
                </div>
                            </div>
                <div class="artist-name"><?php the_title();?></div>
        </a></li>
<?php endforeach; 
wp_reset_postdata();?>

Answer 1

If I assign a featured image to any post in question then it does show the correct image. I just don't understand why every post after the first twelve are using another posts featured image.

I think the problem is that you do not reset $image if the current post doesn't have a featured thumbnail.

You should add for example:

<?php 
if (has_post_thumbnail( $post->ID ) ): 
    $image = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), 'single-post-thumbnail' ); ?>
else:
    $image = array( 'http://example.com/default.jpg' );
endif; 
?>

since you are using $image[0] in your code.