I’ve managed to reproduce the behaviour!
What’s happening is that the back-end subscriber simulation checks for people who have decided to unsubscribe rather slowly. If you have several thousand subscribers, it can take perhaps a minute for it to notice that somebody has chosen to unsubscribe and actually purge them from the simulation.
Ordinarily, that’s no big deal. But what you’re seeing here, is that there are a couple of ways for someone to choose to unsubscribe which don’t actually log out before unsubscribing. When that happens, the subscriber’s character remains on the map, selectable, but doesn’t animate at all. Eventually, the character gets removed once the back-end system notices that the subscriber is unsubscribing and cleans up… but there can be a long time before that happens!
I believe I’ve fixed this now; subscribers now always log out before unsubscribing, which gets rid of these phantom characters.