Diff in PR code review doesn't highlight changes

[JesperIRL]

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Sometimes in PR diffs the changes aren't highlighted. The old line has the dark read background, and the new line has the dark green background, but the differences between the lines aren't shown at all.

An example can be seen here: https://github.com/openjdk/guide/pull/97/files#diff-f43200945b93eacb14613755322fd170adbc7ba5905b9349859b23245f80249b

In this change there are two differences, a lower case c was changed to an upper case C, and the word "committer's" was changed to "Contributor's".

Not having those changes highlighted makes it very difficult to review the change, and it makes one wonder - what other changes aren't highlighted in other diffs?

I have seen this several times when editing markdown files. Is markdown not a supported filetype on GitHub? The reason I ask is when I look at the info about this particular project that contains at least 98% markdown, the "languages" meter says 63.1% makefiles, 33.2% CSS, and 3.7% HTML.