Shape grows really dark when viewed from below

I have a shape (a collection of smaller cubes) that is currently lit only with a skylight. However, when viewed from below, and fairly close, the underside of the shape goes very dark.
The skylight doesn’t have lower hemisphere colour turned on, but even set to something light it makes no difference.
There is a post-process volume that does some depth rendering thing (not even accessed in the attached screenshots) and sets auto exposure off (max and min 1)
Adding a spot or any other light directly beneath this shape does nothing to lighten it.

When I set a camera close to the bottom of the shape and start playing you can see it adapt to the darkness (ie. it starts of “normal” dark but then grows darker still to almost black).

It happens with any camera I set in the scene as well as in the editor itself before I hit play.

Any idea what setting is causing this? I’ve spent all day tweaking all manner of settings and nothing I do seems to make a difference (for the better).