Depending how walls are defined, it is often not possible to join internal walls properly. This is because there are default rules that govern the joining of wall layers. A lower function definition will penetrate higher function definitions until meeting one equal to its own or lower. Shown below is the hierarchy of wall layer function properties.
And the way they are applied to the layers of a compound wall:
This wall is used as the outer walls of a structure. Shown below is the structure of a single wall that is used for the internal walls.
The wall layer functions, Structure
One can choose not to have the wall join (disallow join), but this often does not achieve the desired result. One can also modify the section, but this would only be applicable to a detail drawing.
A workaround, and a method not generally covered in references, is to delete the function of the layer. This may at first not seem possible, since an intuitive method does not apply. Instead the following process enables the deletion of the function property of a wall layer.
Edit the structure of the wall (the interior single wall). Select the function property of the layer.
Press, sequentially, <Backspace><Preview><Preview>. This results in the layer having a null entry in the Function field.
Notice how it now joins differently to the outer walls.
While the inner wall now does not penetrate the outer wall finish, it does represent a closer approximation to reality.