Beersmith does not calculate lactose additions accurately when using regular brewers yeast.
However, some strains of wild yeast and bacteria will ferment lactose and the beer will finish a lot closer to what the software predicts with several months of fermentation.
A possible solution is to create a new ingredient category for lactose AND maltodextrin where you can independently specify SG and % yield for an ingredient.
Currently maltodextrin is in the misc category and adds nothing to the SG which is false.