Unless I’m missing something the pool should be lasting for 7 seconds (4 base +3 from Lingering Toxicity) but they expire after exactly 4 seconds.
Edit: Actually never mind, it looks like it does in fact work. It’s just the ground “slime” effect that disappears after 4 seconds. I didn’t notice the faint green cloud that hangs around afterward which I assume is what’s applying the poison. My bad!