This is pure speculation, but from what you guys are saying for a mesh style manifold to work, perhaps there needs to be some sort of resistance when the wort flows through. For example people with SS braiding, and those who use a manifold and grain bag as well, have less problems with a stuck mash. I can't really think why this would be the case.
Perhaps the mesh used in the manifold, lets the wort through to quickly due to it being highly open to letting wort through, so that it means the mash plugs itself harder into the holes, as it allows a higher flow through the holes
Will