Not seeing Gutenberg "blocks" listed in location rules

I copied the example from https://www.advancedcustomfields.com/resources/acf_register_block_type/ verbatim (code and template file) but I don't see the widget when I create a page.

add_action('acf/init', 'my_acf_blocks_init');
function my_acf_blocks_init() {

    // Check function exists.
    if( function_exists('acf_register_block_type') ) {

        // Register a testimonial block.
        acf_register_block_type(array(
            'name'              => 'testimonial',
            'title'             => __('Testimonial'),
            'description'       => __('A custom testimonial block.'),
            'render_template'   => 'template-parts/blocks/testimonial/testimonial.php',
            'category'          => 'formatting',
        ));
    }
} 

Nor do I see "Blocks" listed when I try to create a custom field.

I'm on Wordpress 5.4.1 and ACF 5.8.9

Start with the reported registration snippet and compare its behavior on WordPress 5.4.1 with ACF 5.8.9. Check why the Gutenberg block and the Blocks location rule are absent; the issue is resolved when the testimonial block appears in the editor and Blocks is available when creating a custom field.