Why is my iPhone transmitting in airplane mode?

I switched my iPhone 8 into airplane mode, but it continued periodically transmitting on 1.8 GHz, as measured by an electrosmog meter. Why? Isn't airplane mode supposed to turn off the radio transmitters? Or is airplane mode on an iPhone only a low-power mode?

After enabling airplane mode, I powered the phone off and booted it in a Faraday cage. It still transmitted on 1.8 GHz, despite not being able to receive any cell tower signals.

Solution 1:

The issue might be related to the "persistent settings to Airplane Mode" since iOS 11. This means that I can go into airplane mode and enable Bluetooth or WiFi and Bluetooth or WiFi will remain on whenever I'm in airplane mode:

However, this does not explain why the 1.8 GHz cellular transmitter still transmitted in airplane mode.

I reset network settings and rebooted the phone a few times, now it isn't having this issue.