You're using the same slot and power cables as the 3060 and it has no issue booting?
Power it on with both 6+2's disconnected. You should get a warning on screen about needing to connect those power cables.
Different manufacturers will pin them differently. It really varies by the OEM. For instance Brand A, might use PSU's built by OEM B for their A model line and OEM C for their B model line. Cables are different. Don't ever mix modular PSU cables unless you're positive the PSU is based on the same design. This is usually pretty difficult to determine.
Now typically with a high quality unit, you're going to short a ground on a 12V and it'll just click and power off. One trick you'll see if with PCIe 6+2's, they'll pin the two grounds to one pin on the PSU side. I'm guessing you ended up in a situation where on the ground pins wasn't grounded at all. Typically with the PCIe connectors if you have no power, you'll get a warning, but if you have partial power, you'll just get no display.
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