Elasticsearch commit search seems to have issues on GitLab EE 9.1

Summary

In some circumstances, searching for commits will return an accurate number of commits, but trying to view the actual search results fails.

Also see by a customer: https://gitlab.zendesk.com/agent/tickets/74368

Like the customer, I have experimental repository indexing turned off, so it's not that.

This only happens on the commit search. The other tabs work as expected.

It also only happens on global and group search. Project search is unaffected.

Steps to reproduce

e.g. http://localhost:3000/search?scope=commits&search=a

What is the current bug behavior?

Commits 354079

Couldn't find any commits matching "a"

(this happens with longer queries too)

What is the expected correct behavior?

I should see commits 1-20 matching "a"

Relevant logs and/or screenshots

Possible fixes

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