Child Theme functions.php was not running any functions

I created a child theme of twentysixteen and was trying to add a nav_menu using

function register_menus() {
  register_nav_menu('landing', __('Landing Menu'));
}
add_action('init', 'register_menus');

I tried a lot of solutions on the internet (increasing the priority, overriding the parent function, removing parent function from the hook), nothing worked, it was not being shown in the Admin Panel – Appearance > Menus > Manage Locations.

Lastly, I unknowingly removed the Template : twentysixteen line from style.css header and re-added it. This fixed my problem.

But why? What is the reason behind this behavior.

N.B. I had a function to enqueue the parent style below it, like this

function enqueue_parent_styles() {
  wp_enqueue_style('parent-style', get_template_directory_uri().'/style.css');
}
add_action('wp_enqueue_scripts', 'enqueue_parent_styles');

It was working correctly but not the register_menus() function.