Sounds like a Motherboard bios bug, speak to your manufacturer.
Sounds like the Windows Power Plan for Ryzen - Bug...
I got this Bug too. After a long IDLE on Windows, the clock dosen´t climbed up over 2 Ghz, dosen´t matter what I try, even with Prime95, the clock was stocked on 2Ghz.
After I choose the Balanced Power Plan from Windows, the Clock works normal and changes adaptiv to the CPU min. Load from 2Ghz to max. 3,8 Ghz (or 3,9Ghz Singel Core). .