Can't control Weldment trim

I am having a problem controlling trimming 2 weldment items to each other.

I have 2 angles that I want to miter at 90 degrees to each other.

But no matter how I select them, they are mitered in the wrong direction.

It doesn't matter which order I select them, where I select them, or even what Corner Type I select.

The result I am trying to achieve is to have a short section of angle connecting the two longer ones, with miters at each end.

But this is what I and up with. What am I missing?

Thanks in advance!