ASUS TUF-RTX3090-O24G-GAMING causing system to not POST when both PCIe power connectors connected - but POSTs while not detected in Windows on either connector individually

Picked up a secondhand TUF 3090 (ex-mining) and having a serious power issue.

Symptoms:

• Both 8-pin power connectors connected → system won’t POST at all. No splash screen, no display, completely dead on startup.
• One connector only (either socket 1 or socket 2 individually) → system boots into Windows, but GPU is completely invisible — not detected in Windows Device Manager or GPU-Z
• Fans and RGB spin up when PCIe power is connected regardless of configuration

What I’ve already ruled out:

• Tested with two completely separate PCIe power cables from the PSU, not just two tails from one cable — same result
• PSU is a brand new Corsair RM850e so I assume unlikely to be a PSU issue
• Two Gigabyte GV-N3090GAMING OC-24GD cards from the same mining rig in the same system work perfectly
• No visually obvious burn marks around the power delivery area on inspection

System specs:

• Motherboard: Gigabyte GA-H170-HD3 (LGA1151)
• PSU: Corsair RM850e 850W

Summary: The card appears to be causing a fault condition on the PCIe power rail when both connectors are energised simultaneously, preventing POST entirely. On a single connector it draws insufficient power to initialise as a functional GPU but doesn't prevent boot.

Card is ex-mining. Suspecting VRM or capacitor fault on the power delivery section. Has anyone seen this specific behaviour before? What should my next steps be, or is it a write-off?