[Hardware issue?] PCIe slot completely dead — no NVMe, no network card detected — CWWK U30

Hi all,
I have a CWWK U300E board (13th Gen Soft Router, i5-1240P, BIOS 2.22.1288, AMI PCI Driver A5.01.29) and the single PCIe slot appears to be completely non-functional — no device is detected regardless of what I connect.
Setup: Passive PCIe-to-M.2 adapter (no switch chip, direct pass-through wiring) in the CEMx4 slot.
Full troubleshooting done:
BIOS "NVMe Configuration" always shows "No NVMe Device Found", with two different NVMe SSDs tested
Confirmed both the adapter and both SSDs work correctly on another PC's PCIe slot (detected fine, one showed up as raw/unpartitioned disk as expected)
Found and switched "Chipset > PCI Express Configuration > PCI Express Slot Selection" from [M2] to [CEMx4 slot] — no change
Did a full power cycle (unplugged PSU for 30s) after the switch — no change
Re-Size BAR Support: Enabled
PCI Express Root Port 1: Enabled, Connection Type = Slot
Removed and reseated the adapter card physically after all BIOS changes — no change
Tested a completely different PCIe card (dual-port SFP+ NIC) in the same slot to rule out an NVMe-specific issue — booted Ubuntu live USB and ran lspci: the NIC does NOT appear either. Only the onboard 82599ES SFP+ ports (01:00.0 / 01:00.1) show up. The PCI bridge at 00:1c.0 is listed, but nothing is enumerated behind it.
This looks like the physical PCIe slot itself (or the M2/CEMx4 mux) isn't actually routing any lanes, despite the BIOS setting being correctly changed and saved.
Has anyone seen this before, or is this likely a hardware defect / faulty mux on this board? Any suggestions before I request an RMA?
Happy to provide more photos/logs if needed.