What you were expecting to happen, and what actually happened?
I cast Mist Stalker’s spell on the “Silent Watchers” challenge of Whitehelm. I expected the Archon Statue to have 1 life left and be poisoned. As suggested by the thread title, no poison was applied.
What are the steps to make it happen again?
Use Mist Stalker’s spell
Do you have any screenshots or video you want to share with us so we can see the problem? Attach them to your post!
Not worth attaching anything since it is so easy to reproduce.