Yes. Looks like it's an issue related to Point Lights.
I've shrunk the characters down to a scale of 0.01 (so they're about 2 metres tall in Unity terms) and the point light has a range of 10 and an intensity of 2. At a distance of about 3-4 units from the Skeleton, it seems to light nicely.
At higher intensities/ greater distances, the light causes a sort of pop-in effect, where bones will suddenly go black. If the light gets too close/ too far away, cracks really start to show...
I've set up a test project if you want to take a look. There are two lights, if you move them left-and-right that should show there's quite a marked difference. I don't know if anything can be done about that, or if all lights always need to be in a goldilocks zone to prevent issues?
https://www.dropbox.com/s/jevubguslcif7wp/lighting.zip
Thanks for your help 🙂