Since 5.2 it seems that any AP champion I use does not get their usual 40% AP bonus damage on their auto attacks vs. inhibitors. It seems to work on towers but the inhibitors and I think even the Nexus seem to not be affected which, unless intended, is making AP heavy comps really poor at base pushing.
I tested this out with Karma in a custom game and you're right. The AP bonus damage doesn't apply to inhibitors. My Karma had over 900 AP and only did about 100 damage for each auto attack on inhibitors. It took about 40 seconds to destroy each one. I can confirm that the turrets are unaffected. I was dealing well over 250 damage on turrets for each auto attack.
I'm guessing that this an inhibitor bug. I'm sure Riot will patch it. If not, this would a huge nerf to all AP dependent champions! They would struggle to base siege. Not sure if the proc bonus AP damage from Lich Bane's passive affects inhibitors. I haven't tested that.
