Wordpress PHP mark a custom post type as viewed in post listing

I have created a function that allows users to mark a custom post type in wordpress as viewed. When they mark it, the post_id they have viewed is noted in wp_usermeta in the db.

Here is the code that marks the post as viewed:

function lesson_watched() {
    $nonce = $_POST['nonce'];

if ( ! wp_verify_nonce( $nonce, 'ajax-nonce' ) )
    die ( 'Busted!');

if(isset($_POST['lesson_watched']))
    {
    $user_id = get_current_user_id();
    $post_id = $_POST['post_id'];

    if(!hasAlreadyWatched($post_id, $user_id)) 
        update_user_meta($user_id, 'has_watched_lesson', $post_id);
    else if(hasAlreadyWatched($post_id, $user_id))
        delete_user_meta($user_id, 'has_watched_lesson', $post_id);
    }
    exit;
}

function hasAlreadyWatched($post_id) {
    $user_id = get_current_user_id();
    if(get_user_meta($user_id, 'has_watched_lesson', $post_id)!='') {
        return true;
    } else {
        return false;
    }
}

I have created a function to check if a post is marked as viewed and returns true or false:

function is_lesson_watched() {
$member = MS_Plugin::$api->get_current_member();
// if user has membership AND has watched the video
if(hasAlreadyWatched($post_id) && $member->has_membership()) {
    return true;
}
    return false;
}

On the front end of my site, I have a loop that shows all posts from a custom post type. I am trying to add a span to all posts that are marked as viewed by using the following code:

<?php if(is_lesson_watched()) {
    echo '<span class="fa fa-eye watched_lesson_stamp"></span>';
} ?>

Note: this code is placed within a loop. My issue is that this code seems to add the span to all listed posts in the post type - not the ones that are viewed.