As part restarting the balancer on step up, the migration manager reschedules migrations with entries in config.migrations, finding the moved chunk by searching the latest routing table for each migration's minimum boundary. After a shard key refine, this may lead to comparing boundaries with a different number of fields, which can lead to spurious overlap or failure to overlap, triggering incorrect recovery or an unhandled exception. Instead the bounds should be extended to account for the refined key.