Artifact upload on failure apparently skipped without reason

Summary

I've seen gitlab-runner 10.8.0 apparently "forget" to upload artifacts on failed builds.

Steps to reproduce

I haven't yet determined how to reproduce this problem.

I have a job that runs with:

artifacts:
  when: always

Expected behavior

Normally, even on failed jobs, I see the following output:

<Some Python traceback>
Uploading artifacts...
some-dir: found 13 matching files
Uploading artifacts to coordinator... ok        id=12345 responseStatus=201 Created token=abcdefgh
ERROR: Job failed: exit code 1

Actual behavior

Sometimes, the runner simply doesn't upload artifacts on failure. I see the following output:

<Some Python traceback>
ERROR: Job failed: exit code 1

Relevant logs and/or screenshots

Interestingly, I see this in the raw output:

section_end:1529067561:build_script
←[0Ksection_start:1529067561:after_script
←[0Ksection_end:1529067591:after_script
←[0Ksection_start:1529067591:upload_artifacts
←[0Ksection_end:1529067621:upload_artifacts
←[0K←[31;1mERROR: Job failed: exit code 1
←[0;m

Environment description

This is a private GitLab CE installation with gitlab-runner 10.8.0.

I'm using docker executor with docker-ce 18.03.