Oh God, I worked on this for around 10 hours non-stop yesterday and it drove me nuts. I had the latest bios, 16.9.2 drivers, Gen 1, 4G enabled, Windows 10 UEFI etc... I connected each card individually and they were all working fine. Woke up for more of the same today and to make a long story short I had forgotten to turn off fast boost! Lol, all six cards running fine now.

Z170a Carbon
G3900
3x Sapphire 470
3x Msi 470

Bty, If for some reason people don't trust OP's link the same BIOs is available on the MSI site.

7A12v18---- https://us.msi.com/Motherboard/support/Z170A-GAMING-PRO-CARBON.html#down-bios