This is intentional and for good reasons. Wither skeleton skulls should not be easily obtained, because they are worth quite a decent sum of money, and you can easily make a beacon with the star which is also worth a good amount of money. In order to decrease the number of skulls, a decrease in spawn rates was needed in order to balance the number of skulls, beacons out there. So, yes this was purposely done. Have a good day!