Latitude 5320: NVMe not recognised, but m.2 SATA is OK

Slightly unusual problem : I have a friend's Latitude 5320 (recently purchased on eBay) which refuses to recognise M.2 NVMe SSDs: when running the built-in Dell Diagnostics, it shows 'Hard drive not installed' and gives an error code (2000-)0155 "Hard drive self-test failed, aborted" and recommends replacing the SSD. 
This SSD works fine on another laptop. I also tried with another known-good NVMe SSD , but get same error.  

If I boot from a Windows installer USB stick , DISKPART doesnt detect either NVMe SSD.

If I then put an M.2 SATA SSD into the same m.2 slot in the 5320, it passes diagnostics fine, DISKPART detects the SSD, and I can install & run Windows.

The laptop BIOS is set to AHCI/NVMe in the Storage section (i.e not RAID/VMD ) , and I have tried switching On and Off the two 'Port Enablement' sliders that allow you to switch on and off 'SATA-1' and 'M.2 PCIe SSD' .. is it OK to leave both of these ON ?

Are there any other BIOS settings that could be causing this behaviour ?  I am not that familiar with this newer BIOS Setup interface : more familiar with the old interface used on earlier Latitudes

P.S. I tried updating the BIOS to the latest 1.45.x, but it didnt help..
thanks for any tips or suggestions as to what could be wrong here.