I perfectly understand all your questions because it sounds like something due to a user mistake.
That really surprised me because I checked multiple times that the orientation was correct, that the correct extender card was used, and that the 651 card was oriented the correct way in the edge connector.
I checked for continuity but at this point, I think there might be a short on the extender itself.
I'm looking into making a new set of extenders because the ones I have access to are very beat up.