Sadly it was a known issue since PBE.
It follows a clear pattern (like dianas passive)
If his Phantom hit does perform the proc, you get neither the animation nor the sound(damage/heal still is applyed though).
Now unlike diana, who most of the time has the proc on the phantom hit as she starts attacking with 0 stacks, Aatrox's stacks don't decay while devourer does, so it often shifts the pattern.
Another thing to note is that the phantom-hit is slightly delayed, so if your second AA already kills the target, the phantom-hit goes void(and with that evt your w-proc).
So while it is consistantly, it still should be looked at.
It's a poor experience for both diana and Aatrox to miss out on that animation/sound and thus it should be changed, so they use that animation/sound also on the phantom-hit.
If you look at Xin for example, even if the phantom-hit performs procs the 3ed Q, both xin and his phantom use his specific knoack-up animation.