In mongodb/mongo-php-library#611, a user noted that they could not create a session when connecting to a hidden replica set member in standalone mode (i.e. TopologyType Single, no "replicaSet" URI option). This is due to the fact that the spec prohibits updating the topology's logicalSessionTimeoutMinutes value based on the isMaster response of a non-data-bearing node.

Under normal circumstances, applications would never communicate with a hidden replica set member; however, if an application connects directly to a hidden replica set member it does so with the intention of querying it. Therefore, I would propose amending the definition of data-bearing node to include a hidden replica set member if TopologyType is Single.