Ive seen people post on this, Are you sure its a bug? Have you tried zooming all the way out to see if the Ataru proc is actually showing up towards the top of the mob? It could be you just cant see the number.
When Ataru Form Procs you also get the Blood Frenzy buff if the buff is not on. The complete lack of such a buff for 20 seconds of Massacre-ing an enemy is a pretty strong indicator by itself that Ataru Form is not proccing, since Blood Frenzy is a 100% chance on Ataru Proc and Ataru Proc is a 100% chance on Massacre.