I am having this same issue after updating to iOS 17.3.1 and the update the week before. I noticed all of my calendars from my iPhone 12 went missing about 2 days after updating.
I contacted Apple and they troubleshooted with me over the phone for about an hour. We tried refreshing the calendars, resetting network settings, reloading a backup that was a few days old from iCloud. The Apple associate did confirm that my calendars were indeed synched in iCloud. It took a really long time for the calendar from iCloud to load back on my iPhone
about a week later, my phone did another update to 17.3.1. On Sunday, I noticed that all of my calendars again had gone missing from my iPhone 12. I contacted Apple again and gave them the case number from before. They escalated my issue from senior advisor to the engineers. I am still waiting for them to get back to me (they are supposed to call me tomorrow evening)
I use my calendar for work (I am a teacher) and I have a ton of deadline, appointments, meetings, online trainings, etc that I no longer have access to. I would just like my stuff back.