Custom error pages not served for namespace domains

Summary

/404.html not served even though it exists.

Steps to reproduce

Create a GitLab Pages website with /404.html. Go to /non/existing_file.

Expected

/404.html served.

Actual

Default GitLab 404 page served when using *.gitlab.io, but /404.html served when using custom domain.

Useful links

Public repository with a minimal, complete, verifiable example: https://gitlab.com/error-page-demo/error-page-demo.gitlab.io

Non-existent GitLab Page: https://error-page-demo.gitlab.io/non/existing_file

Page that should be served: https://error-page-demo.gitlab.io/404.html

Relevant documentation: https://docs.gitlab.com/ce/user/project/pages/introduction.html#custom-error-codes-pages