Perhaps its not that the trigger to remove it didn't work, but it wasn't set up right.
Modify the trigger to display some text, like "Removing disease cloud" to see if its really working. I've had trouble with ability triggers in the past.
Get this - its two buffs. Disease, and Disease cloud. Thats probobly your problem, gotta get both buffs.
And, the clouds that linger around are units. Remember that too. Not the cloud on the sick unit, the ones that float around. They act as a kind of ward.