Cannot un-bury the joint

[Gene Davis]

In an inside ELL of my main floor I have a screened porch, its walls are railing walls, post to beam, halfpost ends.

I have tried everything I can think of, but cannot get one of the joints to NOT bury itself into the wall, halfpost and all. It is frustrating.

I can pull the railing wall end away from the corner in plan view, but I have to get it a few inches away to keep from snapping to join the corner. The walls need to be joined in order to make the deck room.

The other end of the deck wall comes into a 135 corner, not a 90, and is not a problem.

What is the fix here?