These snippets accompany the article: https://www.awesomeacf.com/forcing-acf-field-group-json-file-generation-for-all-field-groups/

generate-acf-json-files-for-all-field-groups-on-request.php

<?php

add_action( 'admin_post_acf_sync', function () {

	$field_groups = acf_get_field_groups();

	// Apply our callback to all field groups
	array_map( function ( $field_group ) {

		// Load up the fields on the field group.
		$field_group['fields'] = acf_get_fields( $field_group );

		// Write the local JSON file for the field group.
		acf_write_json_field_group( $field_group );

	}, $field_groups );

	echo 'done!';
} );

generate-acf-json-files-for-all-field-groups.php

<?php 

$field_groups = acf_get_field_groups();

// Apply our callback to all field groups
array_map( function ( $field_group ) {

	// Load up the fields on the field group.
	$field_group['fields'] = acf_get_fields( $field_group );

	// Write the local JSON file for the field group.
  	acf_write_json_field_group( $field_group );

}, $field_groups );