@jcolp yes webrtc is routed via the same kamailio the corrected diagram would be, and I have identified Kamailio is actually editing the contact.
So this I suspect is an issue with a wrongly called fix_natted_contact() function in Kamailio configuration which I need to figure out how to avoid being called in this scenario.
PSTN → TWILIO → KAMAILIO → ASTERISK → KAMAILIO → WEBRTC CLIENT