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When I first built a system on a B450 Tomahawk core temp worked properly (r7 1700cpu).
 
The last bios update I did it started that showing temps on only half the cores thing like you have.
 
I also have a system with a 3700x/Gigabyte x470 gaming 7. The temps are not working right there either on Core Temp, or any other app. They all show up, but all show the same temp and they cycle from 39c-47c at absolute idle.
 
Something is not quite there yet with Ryzen 3000 bios I think.

In case you didn't know...,
 
The M2B socket on the gaming 5 supports NVME only.  Make sure your not trying to use a M2 SATA drive in there.
 
The M2A socket supports both SATA III and NVME.
 
Easiest way to tell is SATA ones have 2 notches where they plug in, NVME have only one.
 
 
NVME

 
 
 
SATA

 
 
 
The sockets will accept either type.
 
 

Evans coolant.
 
https://www.evanscoolant.com
 
It's a waterless coolant for engines. I think it would be great for liquid cooling.
 
Less chance of corrosion / less electrical conductivity.
 
Designed to last the lifetime of an engine.

Check control panel/power options and see which plan it is running.
 
It should be running Ryzen Balanced.
 
If it's not or Ryzen Balanced isn't there go to the AMD website and download the latest chipset for your board.
 
https://www.amd.com/en/support

Just search "power plan" and "choose a power plan" comes up, the settings are there.
 
If you just built, search for "windows update" and check for updates, let them install.
 

I built a 2600x system a little while back on a x470 mobo, had lots a wierd things going on until I got everything updated, latest bios, etc...
 
Now with the system idling and the Ryzen Balanced power plan selected it sits at 4.2 ghz, if i change it to power saver it drops down to about 2.1 ghz.
 
That's strange to me, but the way it's supposed to work.
 
If Ryzen can stay cool, it stays running fast. Mine is only at 40C idling at 4.2 ghz and that's totally stock, no overclock.
 

If you don't have Ryzen master you can grab this: https://www.alcpu.com/CoreTemp/
 
You can use that to check your cpu temps and clock speed easily.  Your cpu should be below 45C at idle or with little load. If it's over 60C at idle you got a problem.
 
Run windows update and let it do it's thing. You can find it by searching.
 
Go to your mother board manufacturers website and download all the latest drivers for your board and install.
 
Just doing those should get things straightened out.
 
 

Here's a utube vid that may help: