[CHOCK]

My thought is since it’s an aftermarket box, there could be bad machining of the threads on the shaft or a mis-boxed nut that could cause issue with tightening.

Yes indeed. These things are possible on anything made by a human, any machine programed/controlled/operated by a human. Stuff happens for sure.
I think we are at 50+ of the same boxes sold we get out of China. Same issue-Nuts are too thick, pitman fit great (has to have the torque) to press the pitman down on the new splines.