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...
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Thorn's description says "Rounds pierce targets and cause lingering damage over time." Says nothing about poison. You'd think a big gaping hole in a machine would cause some problems. Not saying that it isn't intentional, but if it is, it shouldn't be.