I got there in the end.
In the ACF function I had, I updated the post but BEFORE that I created the publicize message (including hashtags) and inserted that into the right post meta:
// add the tags to the post title as $custom_message
update_post_meta( $post_id, '_wpas_mess', $custom_message );
// Update the post into the database
wp_update_post( $my_post );
And it works now.
Thanks for your help!