Leeching Toxins passive doesn't seem to provide life recovery

I have been using the Leeching Toxins passive for a while now, and I wasn't sure if it was working properly or not since I have +life on hit and +life on kill. But after switching those gears out, I am finding that killing poisoned targets do not give me any health back. I have done this both with gas arrow and poisonburst arrow.

Is there a specific requirement that the target needs to be killed under the "+ #% chance to poison" poison stack? Or that gas arrow kills do not grant activation for this passive node?

Would love to hear back on this.

Much appreciated and thank you for the effort.

Cheers