Today unfortunately same critical bug: again Google calendar entries are deleted without warning, when a pipedrive activity is closed. I noticed that the deleting only occurs when the activity is linked with a deal!
Short-term bug fix is again stop/start of synchronisation: this however cannot be the solution.
I am a pipedrive fan, but this is problem is getting very frustrating, time consuming and is a nightmare for sales employees.
Please ensure a reliable synchronisation between Google calender / pipedrive: check logs / creates reports. Alternatively think of reliable third party vendor to ensure a reliable user experience.