more info: which motherboard, which CPU, which software do you use to detect temps, have you installed AMD Ryzen power plan?
Make sure you are running a newer bios, the older bios had a +20 degree offset, if your temps include that offset then your cpu is actually running 40-50 degrees. It is also normal for various cores to go from idle to overclock when they are called to do something, even a background task. Likewise for the core voltage. Ryzen likes to turn thing on and off for power saving, this results in up and down core voltage.
Below is the clocks of my Ryzen, I have however modified the P-States which is giving me a lower rest clock and a higher overclock. Quite normal for the cores to go up and down. My core voltage varies from 0.750v to 1.375v, I am running an offset to keep my ram overclock stable, makes my core voltage a tad higher then I need just to overclock the cpu.
If you are on a newer bios that does not include the +20 degree offset, I suggest you redo your thermal paste and make sure your heat sink is installed correctly.