Rotation in 3D

I'm in the process of learning Metal for iOS for use in a visualizer app. It's been going good so far, but I'm stumped on this issue I've come across.

Just as a test, I wanted to create a rotating prism (rotating in place around it's y axis). I've used some of the code in Apple's "MetalShaderShowcase" sample code as a reference (it includes a spinning cube as one of the examples).

Here is the prism I created (with depth testing turned off so we can see all the sides) using the following vertices:

https://www.dropbox.com/s/neipoazlcm491vg/prism.PNG?dl=0

static const float vertexData[] = {

0.f, 1.f, 0.5f,

1.f, -1.f, 0.f,

-1.f, -1.f, 0.f,

0.f, 1.f, 0.5f,

1.f, -1.f, 1.f,

1.f, -1.f, 0.f,

0.f, 1.f, 0.5f,

-1.f, -1.f, 1.f,

1.f, -1.f, 1.f,

0.f, 1.f, 0.5f,

-1.f, -1.f, 0.f,

-1.f, -1.f, 1.f

};

The prism seems to be rendering just fine (minus the depth testing turned off of course). But when I try to rotate it, this happens:

https://www.dropbox.com/s/esg41j3ibncofox/prism_rotate.mov?dl=0

I'm not sure why it's clipping and does not seem to be rotating around it's own y axis. (Note that depth testing is ON in the video).

Here is how the MVP matrices are calculated:

Uniforms *uniforms = (Uniforms *)[uniformBuffer contents];
uniforms->model = AAPL::translate(0.f, 0.f, 12.f) * AAPL::rotate(tAngle, 0.f, 1.f, 0.f);
tAngle += 0.5f;

static simd::float3 viewEye = {0.f, 0.f, -2.f};
static simd::float3 viewCenter = {0.f, 0.f, 1.f};
static simd::float3 viewUp = {0.f, 1.f, 0.f};
static float fov = 45.f;

CGSize size = self.view.bounds.size;
uniforms->view = AAPL::lookAt(viewEye, viewCenter, viewUp);
uniforms->projection = AAPL::perspective_fov(fov, size.width, size.height, 0.1f, 100.f);

I'm just using convenience methods provided by Apple in their sample code for calculating the translate, rotate, lookAt, and perspective matrices.

And finally the shader:

typedef struct {
    float4 pos [[ position ]];
    half4 color;
    float mod;
} VertexOut;

vertex VertexOut basic_vertex(const device packed_float3* vertex_array [[ buffer(0) ]],
                              const device packed_float3* colors [[ buffer(1) ]],
                              constant Uniforms& uniform [[ buffer(2) ]],
                              uint vid [[ vertex_id ]],
                              uint iid [[ instance_id ]])
{
    float4 v = float4(vertex_array[vid], 1.f);
    float4x4 mvp_matrix = uniform.projection * uniform.view * uniform.model;

    VertexOut out;
    out.pos = v * mvp_matrix;
    uint colorIndex = vid / 3;
    out.color = half4(half3(colors[colorIndex]), 1.f);
    return out;
}

fragment half4 basic_fragment(VertexOut f [[ stage_in ]]) {
    return f.color;
}

Any helps/tips would be greatly appreciated!