As part of the setup of a sharded cluster in our tests, we refresh the logical session cache on the config servers. This will create and shard the config.system.sessions collection. In the fixture setup, we then force a refresh of the logical sessions cache on the mongoS which will call getCollectionRoutingInfoWithPlacementRefresh for the sessions collection and throw if the collection is not sharded.

However, this refresh is not guaranteed to see the effects of the sharding of config.system.sessions. The mongoS has received no stale config error and so has not advanced the time in store of its cache. And it has not had contact with any shards or the config server since the logical session cache refresh on the config server, so the mongoS will not have advanced its config time. Catalog cache refreshes use kNearest and so there is no guarantee that this refresh will see the sessions collection as sharded.