Apparently, there is a gap behind the first layer of mantlet armor in the ST-I.
The 250 mm mantlet will be split in 2 parts: the 250 mm one and a 270-300 mm one.
The missing armor is almost impossible to see unfortunately.
The IS-4 situation is much worse.
There is a hole in the 170 mm armor, able to be penetrated even by some tier 7 tanks. And many more holes in the mantlet.
The original mantlet armor was splitted again, into 2 parts, with bad armor transition.
Finally, we have the suffering IS-3, which was supposed to be corrected in the 9.8 second test. There is still a large opening around the center area.
Autoloader IS-3 and IS-5 are safe of this bug, fortunately.