After primary steps down and client reconnects, the MongoServer.Primary still points to old primary (that, interestingly, already has IsPrimary set to false). MongoServer.Secondaries properly contains all servers and secondaries.
I wonder if MongoServer.Primary should just return null when there are no primaries after one goes down? I'm not sure how it can become NOT null afterwards though, but then I don't know that much about the driver innards.