In a 2-node replica set, after only restarting the secondaries, the restarted node can become primary, so the connection passed to afterSecondariesHaveRestarted might be the new binary. timeseries_dollar_prefixed_timefield_errors.js failed when it tried to create dollar-prefixed timeField collections on that primary, which the new version rejects. The fixture should pass the node that was primary before the restart (and was not restarted) so tests can run operations that only the old version supports.