After each update, KOrganizer resets the standard calendar to another calendar in my list. That often leads to me adding private appointments to a group calendar which is very annoying. I'm on the openSUSE Tumbleweed packages, not sure if this is a KOrganizer or openSUSE issue.
In my case this reset even happens after each restart what means that the two CalDAV calendars are unselected and Birthday/Anniversaries is selected. I'm not sure if this is a regression, but remember that there was a similar bug that somehow was fixed. At least it did not occur on my machine until the recent update. Now I have installed: Plasma: 5.15.0 Framworks: 5.55.0 Applications: 18.12.2 QT: 5.12.0