Hey, you could try to wrap a conditional statement around the <?php if (has_post_thumbnail()): ?> block. That will specifically affect the post type ‘post’. Because now the loop.php file is probably being used for all content types.
It should look something like this:
<?php if (!is_singular( 'post' ) {
//the thumbnail code
} ?>
This renders the thumbnail code on all post types singular pages except for the posts. I haven’t tested it. Let me know if it worked.