Maybe I'm wrong but the contact shadow artifacts are probably due to GI from the Matte material, which bounce back onto the matte plane etc., maybe even producing the 'look-through' effect since it's screen mapped. I guess if you use a RaySwitchMtl on the matte plane and use a different Material for GI it could probably solve all problems.
I think it always worked like this. It's not wrong either if you think about it, MatteMtl needs to look 100% like the source image which needs self illumination, which then again sends out GI and needs the RaySwitchMtl to override GI.