I have to second jshaw in the first place.
What else to say... It is not a wedging because the post is split above mortise even more than below and goes all the way up. From the pics it is obvious that girt does not twist and the way it checks below mortise is typical moisture movement. I think the post was green when cut.
As to stabilize beams, there is only one way. Proven by centuries. Log your trees in winter, bark them, and season them for at least 6 months. Then you can mill them. 75% of moisture content movement will happen in those 6 months, providing it was barked
Michal
