Panzer IV shots bouncing while flanking IS-2s, KVs, ISUs or Churchills is a common thing.
Agree, that should have been patched a while ago. Not sure why some Allied heavies still have such high rear armour values. But I guess it wasn't considered important enough to warrant attention.
The IS-2 has 140 rear armor;
The KV-2 has 120 rear armor;
The Pershing has 110 rear armor;
The Churchills all have 180 rear armor.
The Tiger I has 140 rear armor;
The Tiger II has 150 rear armor.
All stock mediums, both Axis and Allies, have 100-150 pen (except the T-34-76 which has only 80 far range pen). Allied mediums generally have slightly more close range pen while the Panzer IVs have better long range pen. There is no real discrepancy in rear armor protection between Axis and Allied heavies here, with the exception of the Churchills, but they are somewhat subpar heavies for whom all-round durability (a trade-off for being god awful slow) is one of their perks.
There is a noticeable difference between the ISU-152 (155 rear armor) and the Elefant/Jagdtiger (both 110 rear armor), but that isn't really that odd considering their roles and because of the heavy TDs' much higher frontal protection. And the ISU-152 hasn't been a real problem for years.