Never been on the Wild Frontiers server but as a Rising World server owner myself I do recommend that you join a server that uses a plugin called WorldProtection which basically allows you to protect your area so that people cannot build on your land, cannot destroy your buildings, cannot pick your crops , cannot cut down your trees and cannot steal your items while you are away.....
The issue you have happens with other games too but with Rising World there is a solution when you join a server with that plugin installed.
dont need a plugin installed. RW has a built in world protection. this issue arises from bad permissions file setup.