I recently bought a Threadripper 3970X along with a Gigabyte Aorus Xtreme mother board and my CPU temp in BIOS is pinned at -55c. This makes adjusting fan speed based on CPU temp impossible and is a big problem for me.
I have already done all the basic stuff like re-seating CPU, RAM and GPU. I have updated my BIOS and reset it. I have even replaced my motherboard without any change.
So this makes me wonder if it's actually the CPU that is faulty?
The weird thing is that in BIOS my temp is pinned at -55c, but in Ryzen Master the temperature is read correctly.
I'd suggest posting about the issue in the Gigabyte forums, since if Ryzen Master is reading the temperature properly, and other Windows programs are reading it properly, it's not a CPU issue. Since it happens across two motherboards, I doubt it's a motherboard hardware issue.
-55C is down there with liquid propane. Bit flammable but it can cool a CPU fine. Liquid carbon dioxide can handle maybe -35C which is fine for overclocking too.
I see you have already checked for a BIOS update. This could a case for a faulty CPU thermal sensor.