have you seen (is it normal) that transparency don't work with the object himself.
if you put a strap over the cup the transparent part of the strap make the cup behind disapear (can be seen on your picture)
Yes, the polygons are not transparency sorted within the object itself.
Because sorting transparency polygons is CPU expensive.
There are a few tricks to improve this.
In the render part of the object you can disable the alpha write:
// Do not write to depth buffer
alpha_depth_write = false;
mesh = "clothShape";
This will not write the object to the depth buffer.
But this can also result in other artifacts.