This seems like a pretty tricky question. I see that you have an existing support case trying to resolve the issue. When you've (hopefully) found a solution, please post it on this thread for future users to find!
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Yes, I have raised a case support on this issue. Tableau support team suggested us to provide Tableau log files to see whether they can dig out something interesting.
They are quite nice and professional!
In fact, according to my colleagues on that Tableau+BW project(I have been assigned to another one), they didn’t see this bizarre behavior anymore after rebuilding the cube in BW. So they don't want to bother themselves on those sending log files...
Neither did I have no idea why it happened, nor on how it had been fixed...
hahaha well at least it's fixed...