Send registered users email bug

There exists a possibility for a user to never update an author who’s posts (s)he does not want to receive an email about at the bottom of the “your subscriptions” form. In some cases, if the registered user never updates that, (s)he will not have an entry in the usermeta table for s2_authors as the meta_key. Since this is INNER joined on in the query to decide what users receive emails for posts, these users without the entry will never receive an email, though they’ve never opted out of any author.

An example from a live-site in which I’m having this issue is here:

mysql> select ID from users where ID not in (select user_id from usermeta where meta_key = 's2_authors');
+----+
| ID |
+----+
| 27 |
| 28 |
| 29 |
| 31 |
| 36 |
+----+
5 rows in set (0.01 sec)

There are the 5 registered users who ought to have received emails all along, but never were.

Version: 9.2
WP 3.8.1

https://ww.wp.xz.cn/plugins/subscribe2/