hello. The IS-3 is shown here, but some other vehicles have similar issues.
The issue is that shells are penetrating plates that they should be completely unable to go through.
As shown here, the section of armor plate has around 800mm of effective thickness. the shown shell. BR-471D (yes, I know this is the IS-3’s shell, but it does work with other shells), has an effective penetration of only 227 (this same shell is still able to go through if at 500m) but is still somehow able to get through with no issue. Furthermore, this still creates the spalling. acting like a APHE shell. This was shown from a recent match against an AMX-50 Surbiasse, which does not get APHE. upon further inspection. It looks like the issue is where two armor sections meet in the armor model, a penetration is more likely.
Image of the Surbiasse’s shell able to go through the armor:
Note if you click on the spot highlighted, no spalling will be created. this is likely an error with the armor viewer, because I know from personal experience if you do this same shot in game, the shell will spall as if you shot an APHE shell at it. Again, notice the armor protection-armor penetration disparity. 286mm of pen at 500m, over 1000mm of effective armor protection.