What you are experiencing, unfortunately, is a known issue documented in our list of Known Issues here. The workaround is to immediately refresh after relaunching the app to get the lost changes. We are working on providing a fix as soon as possible, hopefully in our next release of AppBuilder.
Thank you for the patience and understanding.
Check out the Telerik Platform - the only platform that combines a rich set of UI tools with powerful cloud services to develop web, hybrid and native mobile apps.