When setting the FCV to 4.4 from 4.9, we first update to the FCV document, which sets the targetVersion to 4.4. Before we complete the reconfig to change `secondaryDelaySecs` back to `slaveDelay` in the replica set config, stepdown can interrupt the setFCV command. This means that the FCV is effectively 4.4, but the replica set config contains `secondaryDelaySecs` instead of `slaveDelay`. If the node steps back up again (or another node that also has FCV set to 4.4 steps up), then the associated reconfig will fassert because the config is invalid.