This is only a graphical error present only in the plotting of the moment diagram in the Member Query window, to the best of our knowledge. It has since been rectified. The version of STAAD currently in distribution, called STAAD.Pro V8i (also known as 20.07.04.12), plots it correctly. You can download it from the Bentley SELECT services site.
Talking about the build currently in distribution, a new build of the program (20.07.05.13) is expected to be released this week. It has been placed on the SELECT site with a disclaimer in red indicating that it is going through the final certification process, which will hopefully be completed with a day or two.
You can download it now, or wait for a couple of days and download the certified version.