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The person on reddit used a third party cable instead of the one supplied with the device.
https://www.reddit.com/r/nvidia/comments/1ilhfk0/rtx_5090fe_molten_12vhpwr/
It melted on both sides (PSU and GPU), which indicates it was probably the cable being the issue.
12VHPWR is a fucking mess, so please don’t tempt fate with your expensive purchase.
A mini fusion drive, perhaps?