API: `merged_at` values missing for certain Merge Requests in `merged` state

Summary

In gitlab-insights and future features we rely on the merged_at timestamp being available for Merge Requests in the merged state. Under certain circumstances this field isn't available for merged MRs when indexing MRs and retrieving and individual MR via the API.

This definitely does not affect all MRs, but I could not notice a pattern between those affected

Steps to reproduce

• Query api/v4/projects/:project_id/merge_requests
• Notice some MRs do not have merged_at data available

Example Project

The following merged MR is an example that exhibits the problem:

• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24137
• https://gitlab.com/api/v4/projects/gitlab-org%2Fgitlab-ce/merge_requests/24137

What is the current bug behavior?

• merged_at field is null

What is the expected correct behavior?

• merged_at field should be populated with the relevant timestamp

Relevant logs and/or screenshots

N/A

Output of checks

This bug happens on GitLab.com

Results of GitLab environment info

gitlab-ce~3293941 v11.8.0-rc9

Possible fixes

(If you can, link to the line of code that might be responsible for the problem)