Transient: MR added to merge train doesn't get merged and fails silently

As a follow-up to www-gitlab-com!81915 (merged), the activity shows that the MR was added to the merge train on 2021-May-12, however remained unmerged.

On 2021-May-20, it was removed and added back to the Merge Train and it merged successfully.

The purpose of this issue is to investigate the bug and try to reproduce the behaviour from the referenced MR, then propose a solution to solve the bug.