Hi Andrew,
I don’t know the vertex count, but looking at the scene in wireframe mode definitely points to that being the cause of the stepping by way of vertex lighting.
Spotlight properties below.
I did try with the inv.sq function off and, although the lighting is different, the cutoff was still in the same location.
I’m going to see if I can put together a 4.8.3 project in the next couple of days with just this problem occurring and will post it up here for reference, especially now that you’ve pointed out the vertex lighting issue, which hopefully should make it easy to reproduce.
