When using the aggregation pipeline version of updateLookup in change streams, the lookup will inject the document's _id and shard key values into a $match expression, without escaping any potential dollar-prefixed fields.
This allows arbitrary MQL operators to be executed as part of the updateLookup, and can turn the updateLookup from a point lookup into a multi-document lookup.

The document key that is used for the updateLookup needs to be escaped properly so that it can be used safely inside a $match expression.