Why is my iPhone transmitting in airplane mode?
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.