Investigate and understand root cause of Ci::UnlockRefArtifactsOnPipelineStopWorker MR

As part of the Incident Manager process, I am requesting a review of the recent MR that contributed to incident 8619. We should try to understand the root cause and identify learnings that could prevent a similar incident in the future.

Note that we also have an issue specifically to review this incident in its entirety (see: gitlab-com/gl-infra/production#8621 (closed)), this issue is specifically to look at the MR. Please let me know if you have questions or need more details.