Sharded DRM can be deserialized with `metadata=boost::none` because the router already ran elision in its own optimizeAt() and omitted it. However, on the shard optimizeAt() returns early (b/c getNeedsMerge() is always true there) and therefore never re-runs that notification, so the source stage is never told its metadata stream should be suppressed. It then keeps producing a metadata document which leaks into the main results pipeline where $replaceRoot(newRoot: "$payload") fails with 40228 because the leaked document has no `payload` field.