This issue impacts the MongoDB 4.0 branch only.

This following was added which fixes in 4.2 as part of implementing $$NOW and $$CLUSTER_TIME:
https://github.com/mongodb/mongo/blob/fe855c5d116316c50905cf5900c23801fd0c6f68/src/mongo/db/pipeline/expression_context.cpp#L190-L191

Without this copy, a $expr with the "restrictSearchWithMatch" $graphLookup argument will not have access to variables defined in a parent $lookup stage.

When doing this work, we should confirm whether there is identical code in DocumentSourceLookup that can be removed.