Pull request filter does not work

When “Files changed” is selected in a Pull-request, there is a pull down menu filter labelled "Changes from ". I’d expect that if I choose one of the commits in the dropdown list then I’d see ALL changes from thereon to the last commit. But if I select the second last commit then I don’t see changes committed in the last commit.
Moreover, when a specific commit is selected then tab changes from “Files changes” to “Commits” where the specific commit is shown. And if I go back to “Files changed” then “Changes from” pulldown is back to “Changes from all commits”.
Isn’t there any way to see what has been done from a certain point to the end?