posts use index.php instead of single.php

Hello,
I’m (locally) building a custom theme.

I have:

• index.php
• single.php
• page.php
• custom page template called about-page.php
• two different headers for default use and for the single.php

NO functions.php
NO custom taxonomy
Permalink settings > Common settings : default

My index.php has a list of posts (all of those belonging to a specified category), when I click on one of those posts I end to the index.php BUT with the right custom header (header I created for the posts, it’s called header-single.php).

So, I’m getting the index.php istead of the single.php

I read around other people had the same problem with the word “year” or some other Reserved Terms. I don’t think it’s my case.

In my home page I have:

<?php
/**
 * Template Name: Front Page Template
 */
get_header(); ?>
<nav id="menu">
		<ul class="postlist">
			<?php
		        $lastposts = get_posts('numberposts=20&orderby=date&cat=9');
		        foreach($lastposts as $post) :
		        setup_postdata($post);
		        ?>
			        <li><a href="<?php the_permalink() ?>"><?php the_title(); ?></a</li>
		        <?php endforeach; ?>
      		</ul>
</nav>
<?php get_footer(); ?>

Looking forward for some clues.
Thank you very much!
Best,
L