Arbiters always have the downgrade value of FCV, regardless of the FCV of the replica set. So if the arbiter has binary version 3.6, it will always have FCV=3.4. They never learn the FCV, since they do not replicate the admin.system.version collection. Arbiters' behavior is not allowed to depend on FCV for this reason.
We should add a small section titled "Arbiters" to explain that this is expected behaviour since it can unexpected.