Android connects to the phone network using a different type of connection (DUN) to send tethered data.

The code to do that is in android here:

https://github.com/aosp-mirror/platform_frameworks_base/blob...

There is a setting you can set to disable it and make the provider treat all traffic as if it is non-tethered.

    adb shell settings put global tether_dun_required 0

Considering how knowledgeable the HN crowd is on all things networking, it surprises me to see so much uncertainty on something so easy to check in the code!