I would believe the UEFI and ASR's software. Some AMD boards have a so-called CPU temp reading that is impossibly low, as shown in CoreTemp. 13c is ~55F, and it is impossible for any liquid cooler to cool the CPU below room temperature.
The temp in the UEFI is usually higher on most boards because the CPU power saving features have not started yet. Also, UEFI's tend to use one core of the CPU but at a high level for whatever reason. Both reasons are why you see lower temps when in Windows.
I'm a bit confused about where you are seeing the higher temps, is that only in the UEFI or once the OS has booted?
Regardless, can you check the speed of your coolers pump, and the speed of the fan(s) on the radiator. Have they changed for some reason? How are you controlling the CPU coolers fan speed?