ACF Block example radio with default value

acf_field_definition.json

[
    {
        "key": "group_5c375da8e559d",
        "title": "Block Radio choice with default value",
        "fields": [
            {
                "key": "field_5c375e57e0abe",
                "label": "Radio Choice",
                "name": "radiochoice",
                "type": "radio",
                "instructions": "",
                "required": 0,
                "conditional_logic": 0,
                "wrapper": {
                    "width": "",
                    "class": "",
                    "id": ""
                },
                "choices": {
                    "1": "one",
                    "2": "two"
                },
                "allow_null": 0,
                "other_choice": 0,
                "default_value": 2,
                "layout": "vertical",
                "return_format": "value",
                "save_other_choice": 0
            }
        ],
        "location": [
            [
                {
                    "param": "block",
                    "operator": "==",
                    "value": "acf\/radiodefault"
                }
            ]
        ],
        "menu_order": 0,
        "position": "normal",
        "style": "default",
        "label_placement": "top",
        "instruction_placement": "label",
        "hide_on_screen": "",
        "active": 1,
        "description": ""
    }
]

block-radiodefault.php

<?php
/**
 * Block: Radio choice with default value
 * Block slug: acf/radiodefault
 * 
 * @see https://www.advancedcustomfields.com/resources/acf_register_block/
 */

global $post; 

?>

<div class="block-radiodefault block-<?php echo $block['id'] ?>">
		<?php echo "<pre>"; ?>
		<?php // outputs int(2) in the block editor preview, NULL in the frontend ?>
		<?php var_dump(get_field('radiochoice')); ?>
		<?php echo "</pre>"; ?>
</div>

functions.php

<?php
// register acf/radiodefault block.
acf_register_block_type( array(
  'name'            => 'test',
  'title'           => 'Radio Default',
  'description'     => 'Radio choice with default value',
  'render_callback' => 'render_acf_block',
  'category'        => 'common',
  'icon'            => 'star-filled',
  'keywords'        => array( 'acf', 'radio' ),
) );


// render block template
function render_acf_block( $block ) {

	// convert name ("acf/radiodefault") into path friendly slug ("radiodefault")
	$slug = str_replace( 'acf/', '', $block['name'] );

	// include a template part from within the "templates" folder
	if( file_exists( STYLESHEETPATH . "/templates/block-{$slug}.php" ) ) {
		include( STYLESHEETPATH . "/templates/block-{$slug}.php" );
	}
	
}