(ex. Canandaiguan here

)
I wonder if the hardness (or softness) of your water could have something to do with this?
I am not a water expert, but I know the hardness or softness of the water can impact how well soap "suds" or foams right? This could impact the amount of soap residue left on the granules.
BTW we purchased a couple of 2'x4' $5 flouresent "waffle" light covers from HD (the kind that have are made up of squares) and put them around the litter boxes. Not too comfortable to walk on, but we got used to them and the cats did too eventually. This design effectively spreads the cats paws when they walk on allowing the granules to fall through the plastic light covers greatly minimizing the tracking. Trick is to have a confined area where they have to walk on them in order to leave the area. Perhaps you could put them on a couple of your stairs treads (securing them to the tread of course)?
You also have to keep in mind that your cats were probably tracking as much "stuff" w/regular litter, it's just more noticeable with these plastic granules that don't pulverize into a fine dust like regular litter.