You will notice I have regeneration in this video. After my first turn my health does not increase at all on the start of my new turn. On a later turn I cleanse myself by making a match from top to bottom and terror goes away, unfortunately i still don’t get any health back on the start of the next turn. My next turn though, regeneration works as per normal. Throughout all these turns I am also diseased.
It has happened to me too during a PvP battle. Regeneration worked perfectly until being affected with Terror. While having Terror, my regeneration effect would heal for +0. Once Terror was cleansed, it worked as expected. I had dozens of different debuffs during the fight, but Terror was the only one with this effect (even while being the only debuff present).