Does it matter which part of the sandwich is faulty. If it's the thickness of the plywood portion or the density of the foam either way the product loses its rigidity and creates soft spots.
Why the sandwich fails is less important than the simple fact RV manufactures are continuing to use questionable products and questionable construction methods. The consumer ends up holding the bag of soft floors!
I agree sandwich construction is generally found in "Lite" units.But I don't believe it's exclusive to light units.
The real bottom line is units that don't use sandwich construction are generally more costly which makes those units less desirable to $$$ focused consumers.