Posts content is display added with title, instead of content, using the_excerpt()

That’s bizarre. The problem may come from broken html/css with different title or desc lengths. Have you checked the admin to make sure these posts actually don’t have description in their title?
Here is a simple example of showing the title and excerpt in a posts loop.

<?php while ( have_posts() ) : the_post(); ?>
    <h2><a href="https://wordpress.stackexchange.com/questions/282317/<?php the_permalink(); ?>" title="<?php the_title(); ?>"><?php the_title(); ?></a></h2> 
    <div><?php the_excerpt(); ?></div>
<?php endwhile; <?