Inner faces Material ID not working - why?

[itsdavep]

I had a similar problem - inner face material wasn't rendering. I thought maybe because I used a Path Follow it was causing the problem, because 'Bake into mesh particles' didn't fix it, but it did work in a different scene.
I moved the MaterialID into the same event as the Voronoi fracture (right above it) and then 'bake into mesh particles' worked.