Hold guard shields seem to be set up for light armor, using both BashShieldLightImpact and MaterialShieldLight. Given that they are the lightest shields in the game in terms of weight they should probably be light armor. An argument could also be made that they are part of Guard's armor set which is light armor.
Alternatively, if they are to remain heavy armor they should be given the BashShieldHeavyImpact and MaterialShieldHeavy, just like the way Shield of Solitude has been changed.