How to get Secondary Fragments for Uber Arbiter?

So I already had killed him twice, after the 1st time a kind of Tree showed up over the regular Boss tree on the Atlas, now after the latest Patch, this tree is no longer there and no matter how many citadels I run, I only get regular Fragments, not for the Uber Version of the Arbiter, so long story short, how can I get secondary Fragments for this guy?