$facet does not enforce the $$SEARCH_META scoping rules that $lookup and $unionWith implement. Its addVariableRefs does not filter Variables::kSearchMetaId out of sub-pipeline references, and its getSubPipeline returns nullptr because it has multiple independent sub-pipelines, so search_helpers::assertSearchMetaAccessValidHelper never recurses into any branch.

As a result, a pipeline like

[
    {$drm: {source: ..., metadata: {as: "SEARCH_META"}}},
    {$facet: {result: [{$project: {meta: "$$SEARCH_META"}}]}}
]

passes validation and silently reads the out-of-scope variable instead of erroring-out.