Mo, you got it. You've removed material for a through mortise, just where you want there to be strength in the post, and I'm assuming that directly above the mortise you've got a tenon. Because this is a truss, the forces heading down and out (from the rafter) want to lever the top of that post off... the larger the space between the rafter and tie, the larger the forces - and the less wood you have to work with. So its a balancing act. The post can fail at the peg holes, at the bottom of the housing, or if a check develops.

Now, you may have enough meat up there to do what you want - snow loads / wind loads / etc. and size of material vs. span may allow it to work - but I like your other solution better - of resolving the truss loads within the rafter / tie connection, and then letting that resolve gravity loads down onto the post. (then we load it up with wind... and it can get interesting again...)