Just read this from Dell community which makes me wonder. your initial observation is correct. somehow some 5675 motherboard has two implemented m.2 2280 slots for ssd in addition to the short m.2 ssd slot for wireless.  Dell designed a separate m.2 slot for sata and NVMe.  Yet on its manual the 2nd M.2 slot (NVMe) right next to DIMM was not annotated, and not completely implemented on your motherboard by your report. “Slot 1 is at bottom of motherboard - that one is SATA only - and had the original M.2 that I removed after cloning.  Slot 2 is towards upper right of motherboard (next to RAM slots) - this is where I put the new NVME - and has been mentioned by others is the correct slot for NVME support.” This pic proves it.  That is why your NVMe ssd is not detectable in the sata m.2 slot.  The motherboard clearly has two labeled m.2 ssd slots. [IMAGE] if you do not have a completely implemented NVMe m.2 slot, you either have to replace the motherboard or use sata m.2 ssd only on current board.
I have an I5675-A200BLU-PUS desktop, and it reflects there are 2 m2. Ports for pcie upgrades (ssd's) after purchasing an nvme pcie ssd and attempting to install in what appears to be the only functional slot it will not recognize in the bios or windows. What I noticed was of the 2 slots, the one at the base of the motherboard closest to the HDD ports was the only functional slot, since the M2.2 slot next to the RAM slots were missing the actual connector that is supposed to be hard soldered on the motherboard. Can someone let me know if I am missing something on this?
i5675 nvme ssd