I have a pre-comp made of vector shapes which are also pre-comped. All the way down the line, every layer is set to 3D, to collapse transformations, and to cast shadows.
At the very top comp, I've added a light.
When the layer has the collapse transformations switch OFF, the light affects it, but it appears flat. When I collapse transformations, it becomes 3D, but the light ceases to affect it. This seems to go against what I've found on the adobe help pages and what all of the similar posts on this site say.