When viewing events with same UID on different calendars then only the event from one calendar will be displayed.
STEPS TO REPRODUCE
1. create event and invite a person who shares here calendar with you (make it readable)
2. other person accepts invitation
3. let's display both calendars
The event is only on one calendar visible despite the fact both calendars ar visible. It's changeable by disable one calendar and enable it again.
The reason is maybe that the event has in both calenders the same UID.
The event should be displayed on both calenders at the same time.
Linux/KDE Plasma: Arch Linux 5.15.1 (20.08.1)
In work and family environments it is common to make calendars readable by others. It is irritating if events are missing on some calendars.