This is a poison effect and servitors are machines; same with Shanks and you can't poison dot them either. It does all make sense and is surely intended. Vex have a biological mind core so that explains why it does work on them. That's my take anyway...

I think this is intentional. I mean if you think about it, a servitor is a machine and Mark of The Devourer is basically bleed damage. But then there are the Vex...