Issues imported from GitHub do not show up in search results

Summary

Issues imported from GitHub do not show up in search results. In this specific example, the problem is clearly visible when searching for the string "NTP": an issue created in GitHub and migrated doesn't show up in the "All" or "Closed" lists; an issue created in GitLab post-migration DOES show up.

Steps to reproduce

1. Migrate repository from GitHub.com to GitLab.com via the GitLab API
2. Search for a string present in a closed issue title
3. Fail to find the issue

What is the current bug behavior?

Issues migrated from GitHub don't appear in search results, but issues created after migration do appear.

What is the expected correct behavior?

Issues migrated from GitHub appear in search results.

Relevant logs and/or screenshots

Search result (which should show TWO issues, one Closed and one Open):

Existing issue in the project that DOES NOT appear in search results:

Output of checks

This bug happens on GitLab.com