If greater potential energy is toward pos charge, then why PN energy band diagram show lower energy toward pos charge across depletion region?
in an electric field, greater potential energy and voltage are toward the positive charge and away from negative charge

This is correct from the view point of very definition of Electric field.

then why do PN energy band diagrams indicate lower energies toward the positive charge across the depletion region

this is because, in energy band diagram, on y-axis energy is not the energy of a positive test charge (remember, to define electric field, we take positive unit test charge) but the energy of the electron in particular state in the semiconductor.