The problem:

Imperial soldier outfits are divided into three subtypes; light, medium, and heavy. Light is full light armor with a light shield, medium is the light set with a studded cuirass and heavy shield, and heavy is a full heavy set with a heavy shield. However, the outfit lists for Imperial soldiers are set up incorrectly, so soldiers in light armor will always use heavy shields. On top of this, some heavy armored Imperials use light shields, and freed Imperial prisoners during the Battle for Fort Kastav use the medium set but with a light shield. Very inconsistent.

The solution:

Imperial soldier outfit lists have been properly reworked so that soldiers in light armor use light shields and soldiers in medium armor use heavy shields, which is how it was supposed to be. Imperial soldiers in heavy armor that used light shields now use heavy shields, and soldiers during the Battle for Fort Kastav use regular outfit lists.

There is also an addon available to replicate the effects of this mod properly.