The documentation here https://docs.mongodb.com/manual/core/sharding-shard-key/#choosing-a-shard-key doesn't mention that when using a compound shard key, only the first field in the key is used to determine on what shard the data resides. Other fields are used to help improve cardinality, and spread data across chunks, but not across shards.

Since shard keys are immutable, this turns out to be a very important fact.