Queries with an explicit readPreference: "primary" are currently allowed to survive stepdown. This behavior is reasonable when the results are bounded. I.e: some results were returned and the remaining results from a getmore are just as consistent as if the node were still a primary.
However for clients tailing a capped collection (e.g: the oplog), there is no longer a way to guarantee between the driver and server that once a query is opened against a primary, that the node continues to be primary. Applications that desire this guarantee must implement something on their end such as periodically re-issuing the query, or having some side-channel monitoring the replica set state.