Fix for WPGraphQL ACF Gutenberg: block null or duplicated content

function.php

// Add this filter inside your function.php

add_filter('acf/pre_save_block', 'add_acf_block_identifier');

function add_acf_block_identifier($attributes) {
	if (empty($attributes['id'])) {
		$attributes['id'] = 'acf-block-' . uniqid();
	}
	return $attributes;
}

plugin.php

// wp-graphql-gutenberg-acf/plugin.php:86
// We need to replace `$root['attribute']['id']` by `$inner_id` inside plugin

$inner_id = $root['postId'];
  if (array_key_exists('id', $root['attributes'])) {
  $inner_id = $root['attributes']['id'];
}

acf_setup_meta(
  $root['attributes']['data'],
  $inner_id,
  false
);

return $inner_id;