Have you ever tested this with more than one instance? instance_offset_matrix should be instance_offset_matrices and instead of dereferencing it like this, you need to access it using the index i.

Using auto here is simply not acceptable. Unfortunately, it is relatively difficult to deal with these C style arrays, because they can't be returned.
I'll probably have to get rid of the other implicit conversion operators as well, because they cause problems as described here: rB6ac0a3d83c8e5a39bd5356aa0d68e3166bd91e82.

For the time being please use this:

using c_style_float4x4 = float[4][4];
c_style_float4x4 &ptr()
{
  return values;
}

const c_style_float4x4 &ptr() const
{
  return values;
}

This requires you to call .ptr() on the matrices, which isn't super nice, but better than the alternatives that I'm aware of.

There is no need for this type change as part of this patch. Same in a couple of other places.