I'm very sorry for the delay! This is a known issue when hardware information is shared between devices or, in iOS case, when devices with the same OS get their information synced via iCloud. As a consequence, they share the same entry on your account, and, if you save them as Missing, the overwritten device will start reporting too.
To fix this, just go to the Device Management tab on your Prey account, and disable the ‘Identify devices when linking. Overwrite devices if their hardware information already exists on this account.’ Once you save changes, uninstall the app completely from the new device, then reinstall and configure it typing your password with the app open from the device you’ll add to your devices list again.
Please let me know if you need furhter assistance!
Have a nice weekend.