SUMMARY
When a single occurrence of a recurring event is deleted through the Google Calendar web interface, that occurrence still shows up in Korganizer. This persists even after I force synchronization (right click> synchronize > synchronize all) of the 'Google Groupware' component through Akonadi Console.

STEPS TO REPRODUCE
1. Create a weekly recurring event in Korganizer, choosing Google Calendar as the calendar resource.
2. Go to the Google calendar web interface [ https://calendar.google.com ], and delete only a single occurrence of the recurring event (it will provide this option when you try to delete the event).

OBSERVED RESULT
The single occurrence that you chose to delete disappears from the Google calendar web interface, but that particular occurrence is still shown in Korganizer. The occurrence also gets deleted on my phone, so this doesn't seem to be a bug in Google calendar.

EXPECTED RESULT
When a single occurrence of a recurring event is deleted in another device, it should also be deleted in Korganizer.

SOFTWARE/OS VERSIONS
Korganizer version (shown in help > About Korganizer): 5.16.3 (20.12.3)
Operating System: Arch Linux
KDE Plasma Version: 5.21.3
KDE Frameworks Version: 5.80.0
Qt Version: 5.15.2
Kernel Version: 5.10.24-1-lts

ADDITIONAL INFORMATION
Deleting non-recurring events or deleting all occurrences of a recurring event works as expected.