If your events get randomly deleted or even gone completely, there's a 99% chance you are syncing with Yahoo Calendar.
To check if you sync with Yahoo
- Tap the Gears button on the bottom bar
- Tap Calendars
- Check if Yahoo is mentioned here somewhere
Syncing with Yahoo Calendar can cause random deletion of events on mobile devices.
Please be aware that this is not a Week Calendar issue, but a Yahoo issue!
Yahoo gave the following possible solution: https://discussions.apple.com/thread/3186946?start=90&tstart=0.
In short: Yahoo advices to not sync via CalDAV, but to use the built-in Yahoo option instead when setting up a Yahoo account on the iOS via System Settings, see https://weekcalendar.zendesk.com/entries/28862002