As I look at this post checking, it seems to me the tenon of the beam it supports (Hundegger machined) might be acting as a wedge into the mortise. The check or split is centered right up the mortise side of the post above and below the mortise. The shoulder for the beam is not very much over 1" deep. A handcut square tenon could rest in a square mortise without causing a splitting force, but a rounded machine cut tenon unless undersized on the bottom, could act as a wedge, it seems to admittedly non-expert me. Thoughts?
