Is it a PCI 4.0 SSD? My 2TB is only 3.0 and it worked in either slot.Man that is so weird. I’ve never heard of not being able to get into the bios like that. I don’t know if you mentioned it already but maybe Asus might have a bios update to support certain m.2 drives or something. Well good luck and I’d be curious if you find a resolution.
Btw, I also have a Asus Mobo (ROG Crosshair viii hero WiFi x570) and a Sabrient 4.0 m.2. (1 TB) and it works on either slot right out of the box.