Only the latest post shows up on post page?

disclaimer: this is my first foray into wordpress

I'm creating a custom theme and in the single.php file, i have the main loop, however if i navigate to different posts, all that shows up is the latest post. I could go to the different permalinks, but the only post that shows up is the latest one. How do I make it so that the correct post shows up?

I tried changing the permalinks and deleting and adding pages, but I get the same result.

Here is single.php

<?php while(have_posts()): the_post();?>

    <section class="main-section">
        <header>
            <h1 class="section-header"><?php the_category('/');?></h1>
            <ul class="section-nav">
                <?php
                    $cat = get_the_category()[0];
                    $categoryPosts = get_posts(array('category' => $cat->term_id));

                    foreach($categoryPosts as $post): setup_postdata($post);
                ?>

                    <li><a href="<?php the_permalink();?>"><?php the_title(); ?></a></li>

                <?php endforeach; ?>
                <?php wp_reset_postdata(); ?>
            </ul>
        </header>
        <h2 class="section-blurb"><?php the_title();?></h2>
        <div class="info">
            <?php the_content(); ?>
        </div>
    </section>

<?php endwhile;?>

Everything works..but only for the latest post.

Edit:

I reinstalled wordpress fresh and made new pages, but the result is the same. I even stripped the code down to just

<?php the_title(); ?>

in the single.php file, but the result is the same. No matter which post you're supposed to be viewing, it only shows the latest post.