You are likely going to need to talk to someone that knows how the even plugin works. ACF saves fields on the WP save_post hook do_action( 'save_post', $post_id, $post, $update ); and depends on the $post_id to tell it where to save the fields.
There are some possibilities.
The other plugin is not created posts like is normal and this hook is never fired.
The events calendar uses serveral post types and it’s possible that the hook is fired more than once or for some other reason the post_id that ACF is getting is not the actually post that is being created or that you think it is.
Are you sure that the fields are not saved anywhere, look in the DB and search the meta key for the field name to see if it’s saved but for the wrong post.
