I've had issues trying to power a gpu and get display with the 6 vs. 8 pin connector. Or both, just for functionality's sake. Need to figure out which standard they want/prefer. AMD vs. Nvidia.
One or the other, I've ended up with 1x of either as workable, not both, not the wrong one (which ever that is), and then all tends to be right with boot.
It seemed always yet another amd vs nvidia thing which to connect, but almost any time in the past 10 years I've need a psu that supported the 6/8 pin power connector explicitly. Expect the pcie power to fail/glitch/piss-you-off - use external power supply, and a beefy psu at that for any/all that gpu of power.
-mb