can not found somecodes that is really existed in searching page

Summary

I can't search out some codes that are really existed in the repository. for example searching for 'check_merge_requests_available' in 'gitlab-org/gitlab-ce' repository

Steps to reproduce

  1. open 'gitlab-org/gitlab-ce' project home page
  2. search for 'check_merge_requests_available' on the top nav bar.
  3. see the result of searching

Example Project

What is the current bug behavior?

no result shown, error as: We couldn't find any results matching check_merge_requests_available
see: https://gitlab.com/search?utf8=%E2%9C%93&snippets=&scope=blobs&repository_ref=master&search=check_merge_requests_available&project_id=13083

BUT, if I selected some branch or tags (instead of the default master ) it can work well,
see: https://gitlab.com/search?utf8=%E2%9C%93&search=check_merge_requests_available&group_id=&project_id=13083&search_code=true&repository_ref=pl-project-service-json&nav_source=navbar

What is the expected correct behavior?

some results should be shown.

see: https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/controllers/projects/merge_requests/application_controller.rb#L4

What confused me is that: the search page say:

Advanced search functionality is enabled.

beside the 'search' button. but when I opened the link, the document says:

NOTE: Note Advanced Global Search (powered by Elasticsearch) is not yet available on GitLab.com.

and also it was said that 'the Elasticsearch is is not yet available on GitLab.com.' on https://docs.gitlab.com/ee/user/search/advanced_search_syntax.html#advanced-syntax-search-starter-only

I don't know if the advance searching is actually turned on or not. Maybe this function make the searching not 100% exactly.