Most coating manufacturers do not reccommend using any coating under Poly. They may recommend reducing the first coat of Poly, as the seal coat.
Polyurethane is very high in solids, and builds excellent without any type of sealer.
Test both the Shellac and Poly seperately on the same type of wood, it will give you your answer. What your seeing, is a seperation of the the two coatings. It would answer you question about adhesion.