Wrong "This merge request contains no changes"

Summary

A merge request displays "This merge request contains no changes." and an empty diff, even though it does contain changes.

Steps to reproduce

View, for example, lilypond/lilypond!1498 (merged).

Screenshot of current state:

Yet, the tip of the source branch is at this commit:

and the master branch at this one:

The latter is the parent of the former.

I've been seeing this intermittently on that project in the past few days. It happened both with new merge requests and after force-pushing an update (we use the fast-forward merge strategy). Sometimes, changing the top commit (git commit --amend) and force-pushing (git push --force-with-lease) seems to fix the problem, but not always.