Can't resolve discussions on a merge request, no error message given

Summary

I am unable to resolve discussions on a merge request, where I was not the one who opened the MR. There is no indication in the UI as to what the error is, I just click the button and nothing happens. This seems like a permissions issue. This is on the freedesktop.org gitlab instance.

Steps to reproduce

There is a project on which I have no special permissions. A different user forked that project and created a merge request back to the upstream project. He also gave me maintainer permissions on the fork, so I can update the merge request. There are discussions created on the merge request, which I need to resolve after updating the MR.

What is the current bug behavior?

When I attempt to click the "resolve discussion" button, I get a spinner and then nothing happens. When I look at the network tab in the browser developer tools, I see that clicking the button causes a request to be made and the response is a 404.

What is the expected correct behavior?

The discussion is marked as resolved, OR an error message appears explaining why I can't do that.

Results of GitLab environment info

GitLab Community Edition 11.11.0