The connection pooling for read secondary uses a random approach to determine which secondary node to read from. For testing approaches this is non-deterministic.

There is a useDeterministicHostSelection flag, which changes the behavior to round-robin, but is not exposed in the shell (see https://github.com/mongodb/mongo/blob/master/src/mongo/client/replica_set_monitor.cpp). It would be useful if this could be exposed as a parameter.