Healing not Always "healing"
Odd, it could just be you were dealt damage at the time on par with your heal, I know there’s many a time I’ve been downed with no idea what hit me. But your example does make some sense.
I don’t remember what Tequatl and his dumb cousin actually do. But if you’re saying what I think you are, then, well, poison (a condition which only stacks duration) stacked intensity. Since it reduces healing by 33%, taking 3 stacks of poison could reduce healing by 99% or if that 33% is merely a rounding of 1/3, then you get nothing.
Of course, that’s just speculation. I don’t know Arenanet’s coding myself. 3 poison stacks could result in a third of a third of a third or, reducing healing to 8/27 effectiveness for all I know.
“There, it’s dead and it’s never coming back!” – Famous last words
poison stacks duration, not effect. IE: 3 poisons do not = 99% heal reduction, just means if each was 3 seconds long the effect would be exactly the same but last for 9 seconds.