The UpdateStage prevents updating the same record multiple times by tracking in updatedRecordIds the recordIds that have been updated. This structure is updated only when the flag indexesAffected is set to true, but the functions responsible to update it are bypassed in explain mode. As a result, updatedRecordIds is always empty in explain mode, and the _dedupReporter reports zero updates. We should update updatedRecordIds with the records that would have been updated, even in explain mode, allowing for correct reporting of dedupReporter metrics in the explain output.