What version are you running?
2.0.18
What's the URL of the page containing the problem?
http://reviewboard.ad.analog.com/r/8341/diff/2-3?page=1#19
What steps will reproduce the problem?

Upload a diff
Upload a second revision of the diff which makes a small change in a file
Upload a third revision of the diff overwriting the small change and making some further changes to the same file, near where the small change was.

In this case we changed the date from March 13 to November 18 in the second diff, then changed the date to November 19 and removed some lines in the 3rd diff.
What is the expected output? What do you see instead?
If you view the 2nd revision of the diff the output is correct (only the date has changed from March 13 to Nov 18, the lines are still present) and has correct highlighting.
If you view the 3rd revision of the diff the output is correct (the date has changed from March 13 to Nov 18 and the lines are removed) and the highlighting is correct also.
If you view the interdiff between 1 and 2 the differences between the original file and the 2nd revision are shown (correctly and highlighted).
If you view the interdiff between 1 and 3 the diff between the original file and the 3rd revision is shown correctly.
If you view the interdiff between 2 and 3 the date is correct and the file contents are correct (lines shown in revision 2 but not in revision 3), however only the date is highlighted - the missing lines are not.
What operating system are you using? What browser?
ReviewBoard is running on RHEL6u3, clients are Windows 7 using IE10 and Chrome 46.
Please provide any additional information below.
This issue is visible on multiple files in this patch, this one was singled out as an example.