Past events may disappear due to Sync date range settings in the iOS Settings - please rest assured that they are not deleted permanently and can be brought back with some troubleshooting steps.
To set the Sync date range for Local Calendars, do the following:
- Go to iOS Settings.
- Scroll down to the Apps section.
- Select Calendar > Sync.
- Choose All Events.
If something still doesn't work for you after these steps, please tap the Calendars icon at the bottom right to open the chatbot.