Shortcode will not process using do_shortcode

I find that the content_block shortcode will not process (i.e., the shortcode code displays, not the return of the shortcode) in some situations when custom shortcodes I’ve created DO process.

For example, I created a Custom Post Type for which I want the Description (which appears at the top of the CPT archive page) to be created/edited by my client. So, instead of hardcoding the Description inside the function that registers the CPT, I created a Content Block to store the CPT description. I added placeholder content.

I have a function that returns the description, which originally was this…

function seba_directors_blurb() {
	$output = '<p>This is a description of this CPT.</p>';
	return $output;
}

In the CPT registration function, I have this line…

$args = array(
	'label'  => __( 'SEBA Director', 'text_domain' ),
	'description' => seba_directors_blurb(),
	{etc.}

Then, after adding the Content Block for the CPT description, I altered the first function to be this…

function seba_directors_blurb() {
	$output = do_shortcode('[content_block id=1118]');
	return $output;
}

The result is that this appears on the front end:

[content_block id=1118]

However, when I created a simple custom shortcode that returns placeholder text, it works.

I hope this makes sense. Any thoughts?