I was experiencing the same situation with two Hikvision DS-2CD2732F-IS cameras. Everytime the DiskStation would restart the motion detection settings in the camera would be reset to default. I raised a ticket with Synology support and I guess someone else may have as well. As of Synology Surveillance Station 7.0-3742 (released February 2015) there is now an option under IP Camera -> Edit Camera -> Camera Setting -> Event detection called "Keep original camera settings" which does the trick. Have restarted both the SSS service and the NAS with no ill effect on the cameras' motion detection settings.