I suspect it has to do with the rather dumb combined vertex and index buffer type within the PSM API. This causes a proper API like in XNA or MonoGame to jump thru extra hoops to make it seem like vertex buffers and index buffers are interchangeable. See…
https://github.com/mono/MonoGame/blob/develop/MonoGame.Framework/Graphics/GraphicsDevice.PSM.cs#L231
The PSM demos don’t have to jump thru these hoops.
It is telling that this is the only platform where we have this problem.