Created a fighter with 14 Dex, and equipped half plate. AC went from 14 to 20. Equipped a Steel Shield. AC dropped to 19. Unequipped Shield and AC stayed at 19. Unequipped Half plate and AC drops back to 14. Cycle is repeatable.
If I start by equipping the shield, AC goes to 15. Equipping the armor brings it to 20. unequipping the shield takes it to 19.
If I’m doing my math correctly 19 is the correct value. Not sure if I’m missing some nuance of the rules but I think something is funky with the AC math somewhere.