Poison ending for no reason?

Hello! I'm new to this game and playing shadowstriker. I unlocked Deaths Veil for stealth on killing with a focus skill, but most the time it applies it ends immediately. I just see a flash of stealth.

Does DOT break it? If I kill with something other than the last hit of a focus combo, does that break it? I'd like to stealth around, but this perk seems to be mostly wasted.

Thanks!