explain("executionStats") on any pipeline containing DRM tasserts 10422601 "pipeline sizes are not equal" in mergeExplains, crashing the mongod/shard handling the request.
 
For executionStats, PlanExecutorPipeline::writeExplainOps() calls mergeExplains, which zips the two pipelines stage-by-stage and asserts equal length. DRM is a 1→N lowering: it is a single DocumentSource (one serialize() explain entry) but lowers at exec-build time to  DocumentSourceExchange + $replaceRoot (+ $setVariableFromSubPipeline when metadata is bound in-process). The stage counts differ and the assert fires.