I think this is working as intended. When you click on the prow job, the logs you are seeing are pulled directly from the pod. So they are only available as long as the pod is available and pods get GC'd soon after the pods are finished.

For long term storage, Gubernator uses GCS to store a copy of the logs. In the prow dashboard you can click on the job type in the "Job" column to get a link to a prow page like this
https://k8s-gubernator.appspot.com/build/kubernetes-jenkins/pr-logs/pull/tensorflow_k8s/289/tf-k8s-presubmit/355/

If you click on build-log or artifacts that will bring to a page showing the files in GCS which should contain any logs stored for long term storage.