I was having this same problem and it was driving me crazy. I'm not sure if this will work for you, but I just got mine to start working again. I have multiple Gmail accounts and I would share calendar access between all of them so if I wanted to, I could just see all of my events on the same calendar by selecting each calendar in the left column of google calendar under "My calendars". Usually I would only have the main calendar selected on each account, but this made it easy to see all events on one calendar if I wanted to instead of switching back and forth on tabs. Previously this has never posed an issue, however, recently my calendar events stopped showing. All except my co-worker's calendar, which you said was happening to you per your comment on October 6th and it got me thinking, why would there's work and not mine? What's different? Could it be me having multiple iterations of the same calendar in iCal?
So I clicked the Calendars at the bottom of iCal on my iPhone and made sure each iteration of said calendar was selected. This fixed the problem, but it would also then duplicate the event for however many iterations I have of the calendar I had. So I decided to go into each Gmail account and remove access for all my duplicates and now it works again! Each calendar will show an event in the complication so long as there is only one iteration in the iCal app. Does this make sense?
I can no longer see all my calendars in one place on a web browser, which sucks. But at least I have the complication functioning again which I look at all the time. This is still something Apple needs to fix so I can start sharing calendars between Gmail accounts again. This had never been a problem in the previous 7 years of having an Apple Watch and needs to be fixed.