The exact same problem.
3900X + MSI X570 MEG ACE + MSI 1080Ti
Custom EKWB loop. Voltage at full load 1.3 volts. At a partial load and idle 0.9 - 1.5 volts. The temperature in BIOS is ~ 35 degrees. In system while idle, it jumps like crazy from 45 to 65 degrees. 25 minutes of PUBG - 83 degrees. In this case, the water does not reach 40 degrees. The video card does not heat above 59 degrees. Just in case, I completely rebuilt both the loop and the entire system. Clean OS, the latest BIOS and drivers, the right power plan.
It seems that MSI Mobos is receiving and returning the wrong processor temperature. Although it is strange, in BIOS the temperature readings are correct.
Update: And the funny fact, under full load (Prime95), both software shows correct temperature. o_O
I think as per AMD's last Chipset update (in preparation for the AGESA 1.0.0.3abb update) - they changed the behavior of Ryzen Master. Previously Ryzen Master would reported the highest temperature, now it reports an average - which is why it is lower in Ryzen Master.
The nature of the chiplet design and the fact that so much circuitry has been put into what is essentially a very small area - and with the Ryzen 9 3000 series you essentially have two CPUs in a tiny area. So when a core boosts to complete a workload, the temperature in that small area will ramp up very quickly (because the boost is very quick).
As I understand it - this is perfectly normal behaviour, however, the AGESA 1.0.0.3abb will fix a lot of the problems with regard to how the temperature is reported (maybe adding some new metric points?)
I'm still waiting for the AGESA 1.0.0.3abb update for my MSI MEG X570 ACE motherboard, but in the interim, to keep things a little quiet, I've adapted my Fan Curve to be more gentle (but still cooling correctly), and now I see my average temperatures in Ryzen Master (though I can't use Ryzen Master all the time since I enable SVM for development work, and Ryzen Master will not work with SVM enabled 
- please @AMD can you fix this!!! I don't want to overclock, don't need to as the CPU is plenty fast, just want to be able to view what is happening).
Using the MSI provided software or HWInfo64 is sort of okay, but as has been mentioned there is a disparity between the temperatures reported by Ryzen Master and any other software at this time.
I'm keeping my fingers cross that this will be remedied in the upcoming AGESA 1.0.0.3abb BIOS - when MSI actually release it for my motherboard.
Hope that helps.
Thanks 
Also an MSI MB user (MSI x570 Pro Carbon Wifi)
Had something similar happen to me on a fresh Windows 10 install. noticed that one of my 12 Cores had an unusally high CPU usage (Task manager would show constant 5-6% CPU usage at idle, no running progams in the back) causing it to have idle temps of 45-55c with voltages at 1.4xx+ even at idle after installing all the drivers. i tried uninstalling everything i could to see what was causing it without any success. Ended up having to use windows 10s 'Reset this PC' feature (Opted to Remove Everything to make sure it was a fresh install) and after booting into desktop and installing most of the drivers, the problem was resolved. the only two pieces of software i didn't install this time was MSI Dragon Center and Store-MI. i think one of those two may have been the cause of the high CPU usage on one of the cores at IDLE on my windows 10 x64 install.
This is my Dekstop at idle now after doing a fresh start:
Not really sure why PBO Disabled has higher voltage at idle
Cool'n'Quiet has lower Idle voltages but higher voltages under load compared to just having it disabled with PBO ON.
After a fresh install with CnQ disabled, system would idle at about the same voltages as having it enabled but for some reason, after installing all my Apps, voltages would Not go lower than 1.3-1.4 without enabling CnQ in the bios, Atleast thats the case for my Board, other board model/Bios version may perform differently.
Not sure if this helps but posted it just incase.
Did AMD solved that problem or any of the users? I noticed the same problems with the energysaving modes and in idle I have temp. arround 47 - 55 C in games arround 80 C - this is way to high in comparison to my past intel i5-2500k.
My pc setting is similar to the other users:
Ryzen 3900X
MSI X570 - Gaming Carbon Wifi
32 GB Gskill 3200 Mhz Cl 14 Ram
Watercooler - Kraken X72
Help requiered pleaaase
@jamesx380fx Thanks for your quick reply
- I have some more questions to you mate:
How do you messure the temps on your ryzen 3900? With Ryzen master or HW-Monitor?
Do you use any special thermal paste? As we both have the same set up, I am very interested if you changed any settings in the bios which has an influence on the temperature or voltages?
I did some research and found out that the msi x570 gaming pro is not a recommended MBO, because it has high temps on the vrm and maybe its not the best option for a ryzen 9... but your temps look absolutely fine, which means it has something to do with the settings.
At the moment I changed my cooler to an corsair hi150 pro similar to the kraken, but this works much better and I am happy. My idle temps on ryzen master are between 38 - 45 C, you think they are fine? Sometimes they jump in normal windows up to 47 - 55 C. I am also thinking often, that the new MBO & processors have problems to messure the temps, because in the bios I have stable temps arround 32 C, after booting into windows and open Ryzen Master I see the mentioned high temps before.
Many Thanks for all your help and hints.
