Confusion regarding effect of dielectrics on energy stored in a capacitor
It seems like a decreased electric field magnitude would mean reduced energy stored in the capacitor

If that were true, then where did the energy go when the dielectric was inserted?

The energy stored in a capacitor depends on the charge and the capacitance of the capacitor. By inserting the dielectric you changed (increased) the capacitance of the capacitor! Since the energy and charge must remain the same, the voltage must decrease.