Activity app sending E-mails while it should be off

Nextcloud version (eg, 10.0.2): 11.0.0
Operating system and version (eg, Ubuntu 16.04): 16.04
Apache or nginx version (eg, Apache 2.4.25): Apache 2.4.18
PHP version (eg, 5.6): 7.0.13
Is this the first time you’ve seen this error and can you replicate it?:

I have been unable to reproduce this issue with a new user.

The issue you are facing:

Some of our users are receiving E-mails with activity updates send by the activity app that comes with Nextcloud. Most of these users either set it to daily or disabled all the checkboxes so they should not receive further notifications (or so I assume) However the user still seems to be receiving multiple E-mails a day.

There are no related errors to this issue in my logs (Apache and Nextcloud)

The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):

$CONFIG = array (
  'updatechecker' => false,
  'instanceid' => '',
  'passwordsalt' => '',
  'secret' => '',
  'trusted_domains' =>
  array (
0 => '',
  ),
  'datadirectory' => '/var/www/owncloud/data',
  'overwrite.cli.url' => '',
  'dbtype' => 'mysql',
  'version' => '11.0.0.10',
  'dbname' => 'owncloud',
  'dbhost' => 'localhost',
  'dbtableprefix' => 'oc_',
  'dbuser' => 'owncloud',
  'dbpassword' => '',
  'logtimezone' => 'UTC',
  'installed' => true,
  'mail_smtpmode' => 'smtp',
  'ldapIgnoreNamingRules' => false,
  'asset-pipeline.enabled' => true,
  'memcache.local' => '\\OC\\Memcache\\Memcached',
  'filelocking.enabled' => 'true',
  'memcache.distributed' => '\\OC\\Memcache\\Redis',
  'memcache.locking' => '\\OC\\Memcache\\Redis',
  'redis' =>
  array (
'host' => 'localhost',
'port' => 6379,
'timeout' => 0,
'dbindex' => 0,
'password' => '',
  ),
  'loglevel' => 1,
  'appstore.experimental.enabled' => true,
  'appcodechecker' => false,
  'integrity.check.disabled' => true,
  'maintenance' => false,
  'activity_expire_days' => 1,
  'ldapProviderFactory' => '\\OCA\\User_LDAP\\LDAPProviderFactory',
  );

Were using system cron for our background jobs.

I hope someone has an idea of what problem we may be experiencing.