Calendar not syncing across multiple Apple devices

My wife and I each have a Mac (MacBook Pro/Mac Mini), a 2024 iPhone, and an iPad. We share calendars. The calendars have been syncing flawlessly across all these devices until a week ago. Suddenly whatever is entered in the calendar on any screen stays on that device. If either of us makes a calendar entry it doesn’t show on any other screen.

[Re-Titled by Moderator]

Original Title: Calendar doesn’t sync