If your field is a relationship or post object field the only thing that is saved in the DB is the post ID of the related post. When you use get_field() ACF does a query to get that post. If that post is changed then the change should already be reflected. If thee post is deleted then ACF should return nothing. However, if the post is in the trash ACF will still return post because it queries by the post ID and not the status.
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