Several PHPC tests are failing because the first outgoing command executed no longer includes a $clusterTime field:

• causal-consistency/causal-consistency-012.phpt
• session/session-002.phpt

I'm curious if this is related to CDRIVER-4171, if we suppose that the initial cluster time would be collected from the hello response at the same time the service ID would be validated. If that's not happening, then perhaps these outgoing commands are being assembled before that hello response has been analyzed (and the cluster time collected).