I have to walls intersectiing in L shape.
The horizontal one is of a lower height
Problem apears in 3D
The intersction line goes down to the base of the wall
The only solution i found is break the small horizontal wall
and so create a reactangular wall
You could make them both the same height and overlay an extra wall to add the height onto the taller wall. I'd suggest putting this on a 'Model Elements' layer, that is turned off in your floor plans so that it doesn't interfere with your windows & doors.