I have this sheet metal part and I want to weld ONLY the section shown in red.

Using Sheet metal tab - Welded Corner gives me the following result. 

Why SW can't understand (or I'm not able to tell the software) there's no point in welding the section shown above. 

How can I weld only the section shown in image1?

A sample file is attached if you need to see what's going on.

Any kind of advice is appreciated.