Check that the m.2 heatspreader isn’t causing the m.2 drive to flex. I swear I read about another user that had this issue and had to replace the thermal pads but for the life of me I can’t find the source. I thought it was Reddit, but it may have been elsewhere. I’ll keep looking.
I'm not the person you may or may not have read about, but I observed that the lower M.2 heatsink caused the drive to bend. I solved that by simply not tightening the screws completely, rather than replacing the thermal pad.
Good enough for me. Search is over.
Thanks a lot for your replies all, really appriciated. I will check when I'm back but if you've experienced the above also then I assume that's the problem. I also did a wiggle of the heatsink yesterday and that would also cause it not to show in the BIOS or Windows. But ever since I lightly screwed the heatsink in lightly, it's been fine. Will of course check again tonight. For reference I also screwed the SSD in with the standoff and screw on the bottom m.2 slot.