Diff batch loads overwrites previous diff files
When diff batch loading has to load an extra page it will overwrite the data that previously been set with the new data that has loaded.
From what I can see it comes down to https://gitlab.com/gitlab-org/gitlab/blob/master/app/assets/javascripts/diffs/store/utils.js#L220 which says it combines the diff files but doesn't return the previous files, it instead returns the new files passed in. Which in the case of diff batch is the latest page.