Inner edges in alphaShape

I am working on a scrip where 3D-Objects should be generated from point clouds to later convert them to STL-Files. For the conversion from a point cloud to 3D-object I'm using the alphaShape command. Right now I have difficulties with sharp inner edges of the Objects because alphaShape creates some kind of connection between the surfaces so that there is no right angle possible.

Here is a small example to show you the problem:

[x1,y1,z1] = ndgrid(0:10,0:10,0:20);
[x2,y2,z2] = ndgrid(10:20,0:10,0:10);

P = [x1(:) y1(:) z1(:);
     x2(:) y2(:) z2(:)];

 shp = alphaShape(P);
 plot(shp);

I tried to adjust the alpha value and to create the 3D-body using the boundary-function without getting better results. Is there a way to set some sort of maximum value of a point distance which should be connected by alphaShape? Is there any other way to solve this problem or is it just how alphaShape works?

Thank you for your help!