😲BLENDER 3.1 | Cycles with REAL CAUSTICS?

I have downloaded the build - but can’t get the caustics to work.

I have a basic scene with a glass ball, ground plane and point light.

I have set the glass ball to be a caustic caster, the plane to be a receiver and the light to be a caustic light - exactly as per the video, but I don’t get any caustics using either CUDA, Optix or CPU rendering (and yes I have caustics turned on in the render panel).

Has anyone else got it to work - is there another setting i’m missing?