Before completing the transition from catalog shard mode to dedicated mode, all sharded collections that exist on the config server locally should be dropped locally. This is to prevent their collection options and indexes from getting out of sync, since collMod, createIndexes, etc. won’t target a dedicated config server. If the config server transitions back to catalog shard mode, it will copy the necessary options/indexes when receiving the first chunk for a collection.

EDIT: We can't add a the config server as a shard if it has a local database that conflicts with a database in the cluster, so this isn't necessary for correctness, but it does let a user add the config server as a shard after removing it without manually dropping its empty local collections.