The mog house is located at 0,0,0. Some zones have a 0,0,0 in their real geometry that's accessible, some don't. Sometimes it's under the floor, sometimes it isn't.
I imagine the person showing up is related to some missing data resulting in their coordinates being empty. This is further supported by the fact they showed up as Hume M 1A with no gear and the new character flag (if you look up those names, they aren't actually new characters nor are they humes). Zeroed race/face/gear would show that model. Zeroed flags would show new character. Would be interesting to see a packet capture of 0x0Ds when it's happening.
They disappear because the server prioritizes updates on the current target and when you get their real position they're no longer in your mh.