I’m not sure exactly what changed, and when, but in the Time/Date section of an event, we used to have the ability to add additional days. We frequently have one event that starts, for instance, on 9/1 through 9/2, then picks up on 9/5-9/6, but it’s the same event. We entered that as a recurring (I think), and that made the same event show on 9/1, 9/2, 9/5, and 9/6. That option now appears to be gone.
Our Events Calendar Pro plugin had been deactivated (the normal Events Calendar plug was still active though). Reactivating the Pro plugin fixed our issue.