If the random migration fail point is on, it will randomly move unsharded collections in each balancer round. The balancer checks unsharded collections before sharded ones, and each shard can only be used in one operation per round, so if there are enough unsharded collections, all shards will be selected for moveCollections before checking for chunks that need to be moved from draining shards, preventing sharded collections from being drained in suites that rely on this, like the config transition ones.