Pretty sure any stack of poison count as one debuff, thou I'm not 100%, I'm fairly sure.
However, it is the one of the lowest priority debuff, which means healer will remove ANY other debuff over poison.
So it's very easy to protect your poison by hiding it behind a layer of something else, the most common of which is mass ignition from mei.

I know hard cc (immobile/chaos) have highest order, then I think it's acu, then paralysis or blind, then it's ignition. Not entirely sure about tag, that might be the only thing lower than poison, or might be higher than poison.