My enclosed shortcode is setting the content parameter to an empty string

I’m registering a shortcode with the following:

class Resource extends Shortcode
{
    public function action($atts, $content = "broken")
    {
        $markup = "<span id='examp'> $content </span>";

        return $markup;
    }
}

$resource = new Resource;
$resource->register();

in $resource->action(...) $content is being set to empty string regardless of the value in the shortcode.

The full code samples can be found for Shortcode and Resource

• here and here respectively.

I’m having literally no other problems with this. The $attr parameter is exactly as it should be and the only thing displayed is the $content value with no additional markup.
If this isn’t enough information I’ll share the repository – just let me know if it’d help.

Example input:

asdf[resource_download]abc[/resource_download]asdf

Expected Ouput:

asdf<span id='examp'>abc</span>asdf

Actual Output:

asdfabcasdf