Displaying diffs between two versions of an MR doesn't seem to be working properly

Summary

I have a WIP MR where I'm using lots of force pushes. the UI shows I have 9 ("latest" + 8) versions. Displaying the diff between version 8 and version 7 shows the diff against master.

I've just noticed that the versions 1..8 don't have ticks against them in the second version selector. Maybe that means they can't be diffed against? But why?

Greying them out and disabling clicks might be more appropriate than absence of a tick if that's the case.

Steps to reproduce

https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/merge_requests/317/diffs?diff_id=934574&start_sha=55164853d1ea3f2e3610560ac3d6c6cbddfbb632

Expected behavior

Should see single-digit numbers of hunks, or at least a notification or error saying the diff couldn't be generated, here's master instead.

Actual behavior

The diff against master is displayed with no error text