Is AMD35 Pak open top or not?

So I shot the guy twice with 75mm HEAT, straight in the side and… nothing. 1st shot killed 2 guys in the turret. 2nd shot did nothing (as visible on the screenshot, I was trying to hit the ammo and was also still hoping for overpressure). then he was able to hide, peek me it was over. Vs M10, which is A LOT better armored, 75mm HEAT anywhere kills the vehicle instantly. What’s the deal. Is AMD35 a lot better armored than I think?