Hey Ken

My first impulse on water intrusion is to look higher up for roof, trim and gutter faults. I would guess that somehow water flow has a concentrated path to the corner post and is revealed at the lower level. I would look for building features that can collect and deliver water to a point. For example, I have seen plates decayed into canoes and decayed water table trim at the gable tie level course water in a lateral path to a vertical member.

Roger