The Companion app sometimes doesn’t show all events. I have encountered this before but not in such a clear manner: I started the Companion app (iPhone 17, latest iOS), clicked on Big Spin in Challenges, then Find an Event, selected Sunday, and the 11:30 (UTC) event was just missing.
I easily found it on https://zwifthacks.com/app/events/ and registered there but no amount of stopping the app and starting it again made the event show up in the Companion app.
Reset your filters
The Filter button top right is orange indicating a filter is applied. That can happen automatically if you click a link that redirects you to the events in the companion app.
I described the repro steps:
I started the Companion app (iPhone 17, latest iOS), clicked on Big Spin in Challenges, then Find an Event
Of course filters are applied, it is literally because I clicked on a button within the Companion app. Besides the available filters should not filter out the 11:30 instance of the Big Spin, there are no filters available to filter like that.
Where are you located? Is it possible you are seeing the effects of the time change in Europe this weekend?
Iceland; we are on GMT all year round.
Interesting. I am seeing events every hour, on the half hour, in my ZC app (with no filters set). Any chance you see the missing event if you remove the filters? I wonder if whoever set up that particular occurrence just forgot to tag it properly. 
Probably too late to matter now, but here’s a thread that might explain the issue you encountered:
The second post in that thread could shed some light.
I’m in Iceland; we are on GMT all year round. We don’t do that daylights savings nonsense.
I understand that. I am saying that if the event is set up such that it doesn’t appear correctly for you that might be the issue. It’s in the past now, anyway, so it’s moot.
