I’m running Big Sur. My wife and I share some calendars.
today, she has events showing in a calendar she shares with me. It’s not showing up on my Mac Calendar app nor my iPhone Calendar app.
Last week I had an event of my own in a shared calendar. It was visible on my Mac, but not my iPhone.
I tried turning the calendars off and on on devices. no effect. I suppose I could log in and out of iCloud accounts.
Anybody else seeing stuff like this? Any suggestions?
My Big Sur calendar has been complaining about issues the past two weeks or so. The next day in the early morning the problems have so far vanished. Until they re-appear a couple days later.
I have Calendar set up to use both iCloud and Google accounts. Neither Apple not Google have fessed up to having issues with their calendar servers. So 
It appears that events created or changed on an iPhone do not sync to iCloud at times. disconnecting iPhone Calendar from iCloud then reconnecting it seems to fix it for the event.
No idea if it will happen again. This has happened on my iCloud and wife’s iCloud for shared events
annoying.