PC
Playing as Druid
Using Raging Shred, Rabies, and Poison Creeper active and passive, I apply poison to enemies. Every time I watch the health bar, I see a section of the health bar turn a darker color to indicate the amount of poison damage being done. If I leave it alone, the poison ticks down through the entire shaded area of the health bar. If I attack the enemy with Maul (werebear skill) to activate Changling’s Debt, the shaded section of the bar goes back to being fully colored red, indicating that the poison has been removed. It does NOT do any damage or remove the shaded section of the bar. It simply cancels any remaining poison damage.
Also, the passive damage from poison creeper seems to never get affected by changling’s debt in any way.
I have not watched 100% of every interaction to know if this is every time, but every time I have watched the health bar to see if it is working (probably on 15-20 monsters ranging from white normal enemies, through elites, and dungeon/story bosses) I have never once seen Changling’s Debt actually remove the shaded portion of the health bar. So I have never seen it work as intended.