All stones in their smooth and brick forms go to Forester’s backpack from forestry (which is for wood, saplings, seeds etc.) as opposed to going to digger’s or builder’s backpacks.

Well I checked my Code but that Code says it goes into the Diggers Backpack and the Builders Backpack. Whatever the cause is, I do not think that is fixable on my end, and i vaguely remember Forestry screwing up something with their Backpack designation detection system.