[abenedik]

That is strange.

The following code works correctly:

Code:

Point3DCollection coll = new Point3DCollection(new Point3D[] { new Point3D(0,0,0), new Point3D(100,0,0), new Point3D(100,100,0) });

            GeometryModel3D polyModel = Line3DFactory.CreatePolyLine3D(coll, 12, Colors.Red, false, Ab3d.Common.Models.LineCap.Flat, Ab3d.Common.Models.LineCap.Flat, MainViewport);
            Group1.Children.Add(polyModel);
            
            GeometryModel3D lineModel = Line3DFactory.CreateLine3D(new Point3D(100,100,0), new Point3D(0,100,0), 12, Colors.Blue, Ab3d.Common.Models.LineCap.Flat, Ab3d.Common.Models.LineCap.Flat, MainViewport);
            Group1.Children.Add(lineModel);

Maybe you have wrongly set the last parameter: Viewport3D / Visual3D - if you have specified different Viewport3D or Visual3D than the transformation that transforms the 3d space into screen space gives different results and applies thickness differently.