I have a series of comps where the background is set to white in "composition settings". If I lower the opacity of a layer in one of these comps, it fades to white as expected. This behavior holds true when rendering from AE.
However, if I render the same comp through AME (CC), the layer fades to black. I've been able to work around this by adding a white solid layer below the rest of the layers in the comp, but this definitely seems like an oversight on Adobe's part.
Hoping to see this fixed in a future update!