As far as I know this is 3DGS's especially outdated, to an almost unbelievable extent useless and, after all, damn slow stencil algorithm.
I don't exactly know how they work - however this has nothing to do with the models origin I think. The only way to "workaround" this problem is to have the camera at a greater distance to the model and -if possible- not looking at it from the direction the shadow falls to.
JCL once stated that a new, faster algorithm was to be implemented in future versions, but for the moment you'll have to stay with things as they are, I'm afraid...

EDIT: Hey that was someone faster than me again