I have analyzed your model in STAAD.Pro SS5 (build 20.07.10.66) and SS6(build 11.45). The Mz diagram that is displayed for load combination 18 in both the version is --
You have not mentioned any member number for which you have received different design results in SS6 version. I have compared the design results for member 37, 43 and found that the difference is due to different section classification in SS6 version. We will further investigate this issue and let you know.