Given a shard key {"a.b": 1} on a collection split at {"a.b": 100}, across 2 shards.

This works:

insertMany([{_id: 1, a: {b: 50}}, {_id: 1, a: {b: 150}}])

... but this fails due to a duplicate key error:

 insertMany([{_id: 1, "a.b": 50}, {_id: 1, "a.b": 150}])

This is all fine: it means that the routing doesn't recognize the literal a.b field; it only checks the embedded document.

If, however, you open a change stream and insert this document:

{ "a.b": 50, a: {b: 150}}

... the documentKey in the insert event will include "a.b": 50.

Thus, the change stream is saying that the shard key indexes on the literal a.b field, which is incorrect.

Seen in 8.0.12; this may be a longstanding issue, though.