$search / $vectorSearch as extensions expand roughly to:

• $_internalDocumentResultsAndMetadata wrapping $_extensionSearch
• $_internalSearchIdLookup (unless returnStoredSource)
• $rankFusion / $scoreFusion then ask the expanded pipeline: is it ranked? scored? all selection?

Those answers live on $_extensionSearch’s AstNode properties (sortKey, searchScore/score, isSelectionStage) — and 2.0.1 does advertise them. But fusion only looks at the top-level expanded stages. DRM does not forward ranked/scored/selection from its nested source, and idLookup is selection-only (not ranked/scored). So fusion sees “not ranked / not scored / $search not selection” and throws 12108702, 12108712, 12108704, or 12108713 — even though the inner extension stage is correctly marked.