This is the case for any Bluetooth microphone headset, it has nothing to do with the hearing aid “trying to mimic AirPods “it is because Apple refuses to give us fine-grained controls on our Bluetooth devices

Depends if the hearing aid presents itself as an audio sink (with no mic) or as a headset with mic. The phone will only use the mic if it's available (which is generally what you want if you're using an actual headset).