It is odd that your Apple OUI device is joining/re-joining, but the ZD/AP log doesn't show a clean disassociate from the client. This appears to be a client side issue. Is there a later driver for your type of device? Do other clients of the same type have similar problems? Do you have other client types that do not have similar problems?
Not to totally rule Ruckus out, what version firmware on what (ZD managed or standalone AP) are you running?