A constructor that copies DocumentSourceUnionWith accepts ExpCtx for an override:
https://github.com/mongodb/mongo/blob/dbf9b1429b1fbe900472195dd7353cfde835dba3/src/mongo/db/pipeline/document_source_union_with.h#L103

But it doesn't pass it when cloning underlying pipeline:
https://github.com/mongodb/mongo/blob/dbf9b1429b1fbe900472195dd7353cfde835dba3/src/mongo/db/pipeline/document_source_union_with.h#L105

This may lead to inconsistent expCtx between pipeline stage and sub-pipeline.