omg.... It's not inside the wall. ^^ When you create a new material style, it doesn't combine with each other. It seems like ArchiCAD got it wrong. It may seem far-fetched, but there are cases where things like this happen. ㅠㅠ
You are not wrong that there are some problems with the way ArchiCAD merges building materials, and other things too.
For example, using the fillet tool to make a rounded corner.
These are the exact same wall types. But the lines look wider in the curved part and the fills do not align very well.
And I really don't like the segmentation for any Elements in ArchiCAD. This is never going to get any better until they change over to Nurbs.
I don't care if it doesn't make any difference to the people using the Plans - I don't like to see it while I modeling! The segmentation for curved walls (for example) looks terrible in my preferred rendering program.