I was testing in my test world the filtered importerto import and export pickaxes to a arcane bore(yes,to my frame quarry)and when activated the filtered importer, imported all items from the chest and when they entered the inventory they stacked like the screenshots bellow:
in chest:

out chest:

the two ways that I tested i(the results were the same):


I hope that you will fix it.I like very much your mod and the way that you did it :).

Fixed via 24d252d

thanks :)