Currently unable to complete "Get the killing blow on 6 sentients while invisible" riven challenge

[Lord_Khaavren 220]

I have a melee riven with the challenge of "Get the killing blow on 6 sentients while invisible" and it will not make progress despite killing numerous different sentients in different locations with different invisible frames. I tried with Ivara, Ash, and Loki to kill sentients on Lua, inside the hidden laboratories on Jupiter gas city, as well as on the sentient anomaly in veil proxima. The riven still remains at 0/6 completed.

Pictured below is a sentient corpse with the riven progress display showing 0/6 as well as my invisible Ivara that is responsible for it.