I had reported this same issue a long way back, and Benjamin showed me one step I was missing. I can't remember it now of course (the image was of Stonehenge made out of giant Christmas tree lights). I had some grunge decals that were not rendering on the instances. Benjamin showed one simple thing to make it work. Same as this, but now that I set up a test scene, it seems to work. I used the same C4D r20 as before.
Wait, I just remembered the solution. Wardy, make your Decal a Child of the mesh. That should do the trick. I just tested this and it still works for me even when not a child. Let us know either way.
MacOS 12.5.1, C4D r20, Corona Version 12 (Daily Build Mar 11 2024)