The post object only returns the “post” information similar to what is available in “The Loop”. In ACF 5 you can choose to return just the post ID or an object, so that can change things. Either way, the custom fields related to the post object that is returned are not part of what’s returned. You need to use ACF function to get those values. For more information see the documentation for Post Object fields here https://www.advancedcustomfields.com/resources/post-object/
