This is not actually a bug. Soft cloth PhysX in iClone is an Nvidia implementation. And here is a quote from Nvidia PhysX SDK manual:
Collision shapes are specified in local coordinates for the next frame before simulating the scene. An independent and complete collision stage is performed as part of each solver iteration, using shape positions interpolated from the values at the beginning and the end of the frame. Sphere and capsule collision supports continuous collision detection and use an acceleration structure to cull far-away particles early in the collision pipeline. Spheres and capsules are therefore the preferred choice to model the character shape
, and convexes and triangles should only be used sparingly.
Same goes to non-character related cloth, such as blanket for instance. Convex hull might
work for certain types of collision objects as I've been testing it, but could be quite unstable.