Just failed a 5->6 upslot with a 40% booster, after going 5->6->5. That's 95% each normally, which should be guaranteed with the 10% weapon affix boost. Refund my 300 Excubes dammit. And give me an extra Excube to make up for having to re-upslot to 5.
Thread appears to have provided information resolving the problem? I'll put this on hold to not panic people if I'm wrong.