[5e] Sheet adjustment guidance for a medium armor master feat

OK so I'm wearing half-plate, have a 17 dex and will have this feat soon. I've tried to just drag in the feat from the compendium and it doesn't seem to affect either my AC or my stealth rolls. It actually has red lettered words "Heavy Armor" when I roll stealth checks. Please guide me masters! Feat description... You have practiced moving in  Medium Armor  to gain the following benefits: Wearing  Medium Armor  doesn’t impose disadvantage on your  Dexterity  (Stealth) checks. When you wear  Medium Armor , you can add 3, rather than 2, to your AC if you have a  Dexterity  of 16 or higher.

I used the Global AC Modifier to create a "Med Armor Master" modifier with a value of 1. Whenever you're wearing medium armor, check it and your AC will go up 1.