The Blizzard_PVPUI is marked as LOD. Have you tried checking it is actually loaded after you've clicked the stat button and, if not then loading it, before the button does anything else (or before you stat. window loads)?
After loading the PVP UI Blizz. does
Code:
C_Timer.After(1, function() TopBannerManager_Show(PvPObjectiveBannerFrame, { name=bannerName, description=bannerDescription }); end);
Which would indicate you might need a similar timer in order to get the required information from the server into the cache.