Well *hopefully* it's fixed soon but I doubt it, it's been this way for a loooong time now and nothing has changed :( Really sucks because having that 20% is super helpful...
I've been here before (previous alpha) and they said it was fixed in a patch note so either I'm doing something wrong or they are.
Basically the AP ammo says that it should do more damage to armoured targets since it ignores 20% of their armour, but when testing this in 'god mode' with cheats...