I'm having an issue where some of my instanced objects are not casting shadows when rendering to Arnold.

In my example project that I've uploaded, box_instance_1 is not casting shadows on the ground plane where as box_instance_2 is casting shadows perfectly fine. Even stranger, when I hide box_instance_1 from render, box_instance_2 suddenly does not cast shadows.

As far as I know I've set up both instances in the same way. Clearly something weird is going on, and I'd love to know if anyone else has encountered this issue?

I'm on version 16.5.439

switch your Pointinstancing to Fast point instancing
it will work