Indeed, sometimes CAD just won't work as expected... I moved 3dpoly, point at middle of segment, tried several times and at the end it worked... Sometime you just have to nudge things a bit to make it work - that's CAD (it happens from time to time, for ex. intersectwith method doesn't work if entity is far away from origin 0,0,0 - this isn't such a case, you just have to do something on entity that behaves buggy...)