Bug? add_post_meta / update_post_meta

I have a JSON encoded string $json_string which needs to be stored into the post meta data.

Doing the following damages the JSON encoded data:

add_post_meta($post_id, 'json-data', $json_string);

After searching the Internet I found that somebody else had a similar issue, and it looks as though the following addresses the issue:

add_post_meta($post_id, 'json-data', $wpdb->escape($json_string));

3 Questions:

#1) Is this a fault with WordPress?

#2) Is the above safe? Is it likely to damage my clients data?

#3) Could there be a security risk with the default “add_post_meta” functionality?

Kind regards,
Lea Hayes