You can get acf custom field’s value from block’s data.
function get_block_data($post, $block_name = 'core/heading', $field_name = "" ){
$content = "";
if ( has_blocks( $post->post_content ) && !empty($field_name )) {
$blocks = parse_blocks( $post->post_content );
foreach($blocks as $block){
if ( $block['blockName'] === $block_name ) {
if(isset($block["attrs"]["data"][$field_name ])){
$content = $block["attrs"]["data"][$field_name ];
}
}
}
}
return $content;
}