I just checked on some used BM1397AG I have; there is about 30Ω between the bottom GND pad and the top. I also checked on my last remaining new BM1397AG and it's more like 90Ω.
It is my understanding that the S17 has individual heatsinks for each chip because of this problem.
i have seen several refits for S17+ using a single large combed Heatsink plate for several chips all at once ( 3 large heatsinks per Hashboard ) . Not sure , how they get them to work