Overquota notifications stop working [3.2]

After upgrading from 3.1 to 3.2, website overquota mail notifications stopped working throwing this message:

Fri Oct 30 00:00:05 CET 2020 PHP Fatal error: Uncaught Error: Call to a member function mail() on null in /usr/local/ispconfig/server/lib/classes/monitor_tools.inc.php:861
Fri Oct 30 00:00:05 CET 2020 Stack trace:
Fri Oct 30 00:00:05 CET 2020 #0 /usr/local/ispconfig/server/lib/classes/cron.d/300-quota_notify.inc.php(267): monitor_tools->send_notification_email('web_quota_notif...', Array, Array)
Fri Oct 30 00:00:05 CET 2020 #1 /usr/local/ispconfig/server/lib/classes/cronjob.inc.php(87): cronjob_quota_notify->onRunJob()
Fri Oct 30 00:00:05 CET 2020 #2 /usr/local/ispconfig/server/cron.php(116): cronjob->run()
Fri Oct 30 00:00:05 CET 2020 #3 {main}
Fri Oct 30 00:00:05 CET 2020 thrown in /usr/local/ispconfig/server/lib/classes/monitor_tools.inc.php on line 861
Fri Oct 30 00:01:02 CET 2020 finished server.php.


Resolved reverting 861 line in /usr/local/ispconfig/server/lib/classes/monitor_tools.inc.php to previous function call:

for($r = 0; $r < count($recipients); $r++) {
# 3.2 error
#$app->functions->mail($recipients[$r], $mailSubject, $mailBody, $mailHeaders);
# 3.1 works
mail($recipients[$r], $mailSubject, $mailBody, $mailHeaders);
}

I 've a multiserver configuration, "Use SMTP to send system mails" in main config is enabled and SMTP server options configured.
Is it a bug or i'm missing a new configuration?

 

Just to add more info, Mail service is completly unconfigured in my ispconfig infrastructure becasue mail is managed by other systems.

 

Done:
git.ispconfig.org/ispconfig/ispconfig3/-/issues/5896