Hi,
Can you initiaite some traffic to the Internet and capture the session output using get session src-ip <Src-IP used>.
You can read the session information using https://kb.juniper.net/InfoCenter/index?page=content&id=KB24728 . If source NAT is being done, can you enable policy based NAT on the already existing policy from DMZ to untrust and check.
Thanks and Regards,
Pradeep Kumar