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.
Similar experience - just with the RX 5700XT, riser power connected to SATA connector over a SATA->6-pin PCIe adapter, the fans were spining, but only with low rpm, unable to get it spining more.
When connected riser power directly to 6-pin PCIe connector, now the fans are rotating well. Weird, but real.