There was a discussion on tang cracks much earlier than the original start of this one that to me made sense, that was before I even had a though of ever owning an 1899. The tip was to make sure there was a slight amount of clearance at the rear and tapered sides of the upper tang so the stock was supported only by the sides and not by the rounded rear of the tangs. Supporting anything on a rounded or tapered surface creates side forces as you get away from the exact center. Take a couple pieces of wood and separate them slightly and then push them against a rounded surface about the same diameter as the tang and they will spread apart. If you shim the rounded rear of the tang it should be with a thin strip in the center so you do not increase the diameter toward the sides and create firmer contact there causing more side force.
The angle of the stock screw and the angle of recoil forces could be why it only seems to be at the upper tang, or it could just always be due to side force when bumped or dropped.
Ralph had a couple experimental 99's that did not have the step at the rear of the receiver so the sides of the stock were thicker, it was thought that was to lessen the chance of the stock cracking by adding more support there.
