Conquest Sector 4 Deathmark feat

I’m experiencing the same thing. 

I was able to get Death marks earlier in the Sector but they came only from Death Trooper. My most recent attempt was in the Nighter sisters one after Wandering Scavenger. It does trigger as it did on Mother Towsen. But it didn’t count towards the feats. I am carefully planing my usage to get the feat. But it’s become frustrating to waste these attempts.