I had this same this week. My 1660s started to spin the fan after pluging into a real pciex16.
Dont ask why. Just dont know.
First - riser - no spin.
Than pcix16 - spinning fine, nxt step - riser - all fine.
Maybe this will help you too ??

p.s. I was ready to send it over, but seller forced me to plug it into real PCIx16 - helped.