This appears to be a regression (at least I've never noticed it before) and is reproducible:

When a commit with at least two changed files is first opened (it disappears after clicking long enough but reappears when selecting another commit and then the current commit again) and the first changed file is clicked, the diff of that file is shown correctly. When selecting the second changed file, the diff from the first file remains and the diff of the second file is appended below.

Sometimes the bug already appears when first displaying a commit and clicking the first file; then sometimes the diff view doesn't change at all and still shows all changes.

Expected would of course be, that only the diff of the second file appear.