In a sharded environment, we currently add a sort while merging as part of the idLookup stage. However, idLookup is omitted when the storedSource flag is set on the search query, so the results may be merged in a different order. If users expect this implied sort, this could be considered a bug.
It is also worth noting that this blocks running stages in parallel on the shards after the idLookup. If we add this to storedSource, those queries could take a performance hit.