Attack ground on ATGs (like ZiS) make the Panther shells go right through it.
Tanks like PzIV do not have this problem.
Do not attack ground, attack weapon directly.
I honestly didn't know this, just tested it. Attack move on weapon does indeed make the Panther shots connect. I guess even after 500 hours there's still things to learn.
I suppose that solves it. Still, it's weird that the attack ground doesn't work on the Panther while it does on most (if not all) other tanks.