Yeah, that one is a bug. If you look at the Evo/Revo armour model and look at the rear section of the side armour, it is 80mm raw thickness and does not seem to gain any effectiveness from angling against any shot in the game.
If someone could test this in the proving grounds or custom match, it would be very helpful.
Edit: can confirm that this occurs on the Evo in proving grounds, I managed to pen the rear side armour at an angle of over 85 degrees (I had to hunt for pixels to find the spot I could pen, but I could pen it)
With the Type 99:
With the Ramka:
Notice the tracers from the AP shots ricocheting off the side armour after they penetrated and did full damage....