The IndexBuildsCoordinator-StepUp thread attempts to use OperationContext::setAlwaysInterruptAtStepDownOrUp_UNSAFE to ensure that the thread will always run as primary or be interrupted by stepdown when taking locks. However, since the RstlKillOpThread thread runs asynchronously before the RSTL has to be exclusively acquired for step down, it's possible for it to finish running before IndexBuildsCoordinator-StepUp does its setup. Then the step up can complete, thus unblocking the step down. This allows the IndexBuildsCoordinator-StepUp thread to continue as a secondary, without having been killed by the step down.