Search API returns project_id 0 for all results (blobs elasticsearch)

When using the Search API for blobs (global, group and project search) as explained in the Docs, all result entries contain "project_id": 0 instead of the correct id.

> curl --request GET --header "PRIVATE-TOKEN: <your_access_token>" "https://gitlab.example.com/api/v4/search?scope=blobs&search=maintainer%20filename:package.xml"
[
  {
    "basename": "package",
    "data": "<meaningful_data>",
    "filename": "package.xml",
    "id": null,
    "ref": "<meaningful_commit_hash>",
    "startline": 35,
    "project_id": 0
  }
]

Summary

The project_id of Search API requests for blobs is always 0.

Steps to reproduce

curl --request GET --header "PRIVATE-TOKEN: <your_access_token>" https://gitlab.example.com/api/v4/search?scope=blobs&search=installation

Example Project

What is the current bug behavior?

"project_id":0

What is the expected correct behavior?

Meaningful project_ids, eg.:

> curl --request GET --header "PRIVATE-TOKEN: <your_access_token>" "https://gitlab.example.com/api/v4/search?scope=blobs&search=maintainer%20filename:package.xml"
[
  {
    "basename": "package",
    "data": "<meaningful_data>",
    "filename": "package.xml",
    "id": null,
    "ref": "<meaningful_commit_hash>",
    "startline": 35,
    "project_id": 5066
  }
]

Relevant logs and/or screenshots

Output of checks

Results of GitLab environment info

I'm not the system admin, so only info I have is that we use GitLab Enterprise Edition 11.8.1-ee

If that's not enough, I can request the environment info and application check.