Super simple shortcode not working

Have been looking at various tutorials and answers here and elsewhere and just can’t get this to work. Just trying to display a simple output from a shortcode as below, but it is just outputting the actual shortcode tag to the page instead of the content from the shortcode

Functions.php

<?php 
function welcome_roundels_func(){
    return "testing";
}
add_shortcode( 'welcome_roundels', 'welcome_roundels_func' );
?>

add_action( 'init', 'register_shortcodes');

Usage

[welcome_roundels]

Is there anything else I have to do to enable shortcodes, or should the above work?

Edit
I am also using the below code to output the content

$welcomePage = get_post( 5 ); 
echo $welcomePage -> post_content;