Connected RSSI on iPhone invalid

Good Afternoon,

We are using the nRF52840 DevKit with our BLE peripheral app and after connecting on iPhone (iOS11) and trying to read the RSSI (readRSSI() from CoreBluetooth with SWIFT) the values are always invalid, mostly 0. We tried third party apps and the same occurred. When trying to actively read the RSSI on the peripheral side the RSSI is OK.

Is there any reason to justify this? Do you have any suggestion to solve it?

Thank you,