Yep happens to me every now and then. Actually it did last week. I called my TA who could see the dropped segment on her system but she called NZ and they confirmed it was there. It's annoying but there's no way to add it back to the app or to MMB online. NZ say it's a technical glitch that happens when bookings are altered in odd ways but can't give a better explanation than that. But I always call NZ/my TA to confirm it's just a system glitch and that the leg hasn't been cancelled/forfeited.