When I uncheck "Show Reminder Daemon in System Tray", korgac is stopped, therefore I don't get the reminders. This contradicts the explicitely stated Note, that the daemon keeps running, even when unchecked.
*** Bug 421771 has been marked as a duplicate of this bug. ***
Reproduced with 5.17.2.
korgac has been replaced by a new reminder daemon, kalendarac, in version 22.04.0 (5.20.0). Kalendar does not have a system tray entry, and KOrganizer no longer has a setting to control the daemon.