Extension stages that desugar into DRM during LiteParsed expansion carry their DPL as an
in-memory callback supplied by the extension. This callback is not part of the stage's BSON serialization.

When a DRM stage is nested inside a $lookup or $unionWith subpipeline that targets a sharded collection, the router serializes the already-desugared subpipeline to BSON and ships it to the shards. The in-memory callback cannot survive the BSON round-trip, so the shard re-parses DRM without it.

As a result, DRM's distributedPlanLogic() has no plan to offer on the shard:

• document results are not merge-sorted across shards, and
• the metadata stream is not merged, so $$SEARCH_META is not populated.

Top-level (non-subpipeline) sharded DRM is not affected, because the callback reaches the router's DRM stage in-process.

SERVER-128789 fixed callback loss across the in-process LiteParsed handoff. This ticket covers the remaining case where the callback must cross a BSON serialization boundary into a subpipeline.