Why is rotating this prism in iOS Metal not rendering correctly?
The problem was in the shader, and I had the order of multiplication wrong with the MVP matrix. 

So this: 

out.pos = v * mvp_matrix;


should be: 

out.pos = mvp_matrix * v;


For whatever reason, I'm used to dealing with row vectors and had myself convinced the problem was due to the matrices themselves and the clip region..