Posts without featured image using other post’s featured image

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.