From EntityMorph.java in setEntity():

if (entity instanceof EntityLiving)
{
((EntityLiving) entity).setLeftHanded(true);
}

This always sets the mob to be Left-Handed. No idea why it is the way it is.

Oh, that's interesting... 🤔

Judging by blames, I suppose I was thinking of adding support for Left/Right arm skin configuration to the morphs, however, I suppose, I got distracted, and left it as is. I fixed this issue in the code. It will come out with new update on 26th. Thanks for pointing it out!