when you use a mitter join, between two walls of different heights the result is something like this:

I think in every situation this is not correct. The correct should be something like this:

or like this:

or like this:

depended on where the walls were extended by the user.
Right now the only way to achieve this is by creating separated walls, which is a little painful.
I think the developers should consider this in feature 3d wall improvements.