I closed all running apps and preceded with the 10.14.6 install today which, of course, restarted the iMac (Retina 4K, 21.5-inch, Late 2015). A bit later when I opened Calendar none of my calendars showed up. I have iCloud set up to sync the usual Apple apps including Photos to my herd of Apple devices so I checked Calendar on my iPhone and it was fine.
Everything else seems to have been unaffected.
I checked iCloud just to be sure the data was there, and all was fine through the browser interface.
In Calendars I tried to use Calendar > Accounts… and Calendar > Add Account… to force it to see the iCloud calendars and sync them, but was told I was already signed in to iCloud. I tried various dope slaps such as restarting the app and changing the View back and forth between Full Screen and Window, but no luck.
Finally I went to System Preferences > iCloud and signed out of iCloud on the iMac. Of course this meant turning off all the syncing of app data and reassuring the software that I really did want to turn off Find My Mac. Then I just signed into iCloud again. After it finished re-doing everything it had undone, I opened Calendar and all my calendars magically synced again. Problem solved, but what the heck caused it in the first place, and should there be a cautionary tale in here somewhere?
Sorry to hear about all that trouble, Jack. I find it annoying when things out of the blue stop working and the only way to get it back seems to be to stop syncing and restart it. To me typical of the lack of diagnostics and proper recovery procedure for so many cloud-based systems. “Stop syncing and restart it” seems to be the modern-day equivalent of the old “reboot Windows” mantra. I find it not the least bit smarter.
I think Apple makes it a bit harder yet by stubbornly refusing in most apps to give us force sync commands (the exception being Calendar’s cmd-r) and actual cloud diagnostics (“what’s it doing now?”). We used to have some diagnostics through system logs, but ever since Apple effectively broke logging to introduce some obscure new method that supposedly is much better (yet nobody uses), you have to jump through hoops to see what an app or service is actually doing. Regarding force sync, sure in a perfect world you don’t need that, but IMHO I’d like the force sync buttons first, and then once we’re arrive in that awesome world where it’s never needed they can still remove it. For now, we are nowhere close to being there.
To get back to your original issue, I also recently updated my main MBP to 10.14.6 and I have not yet observed any such issues. My calendars are all still there and syncing, both iCloud and Google calendars appear to be working like before.