Dropping a shard-local / untracked collection does not clear its CSS. ShardServerOpObserver::onDropCollection currently leaves filtering metadata in place, so an UNTRACKED (or other) CSS token can survive after the local collection is gone.

This mainly shows up for drops that bypass the sharding protocol: direct connections to a shard, and internal drops via DBDirectClient. Combined with create paths that install UNTRACKED on non DB-primary shards (see SERVER-133352), the leftover CSS is reported by checkMetadataConsistency under 9.0 FCV.

Ideally, those drops leave CSS consistent with the authoritative model, so leftover filtering metadata is not something developers have to remember to clean up or work around.