The moral, when buying "stuff" with obscure Dell, HPE, etc part #s:
"Expect the unexpected." [Boy Scouts (motto:"Be prepared."), meet up with Murphy.]
I suspect that Dell has connected [for each (of 4) target device] (via PCB traces) the (4) PCIe lanes from the switch chip to the SFF-8643 receptacle in a manner that deviates from the SFF-9402 spec. SFF-9402 defines the (PCIe-specific) pin assignments (lanes and CLK) for both SFF-8643 and SFF-8639 (U.2). "Normal" cables should, and usually do, follow that spec.