@ketma hope you get it sorted. I doubt it would be a HW problem.
I've left HWInfo running while using a few apps and playing a few games and the 1950X shows a max speed of 4175 (Hwinfo will show min, av, max MHz hit for each core for the entire period you're had it open).
Have you tried contacting AMD? Maybe look at their forum or contact support.
Only thing I can think of is make sure you don't use the Ryzen power profiles -I had a problem with general slow performance using one in the past although that doesn't seem to be the case here. Might be worth checking the power profit too, to make sure there is no max CPU speed set.
Use Ryzen Master and select I think the Creator mode and apply that (should be already set anyway I think)
Reset bios to defaults then on reboot set your memory up (DOCP) if using memory with XMP profile and see how you get on with that - leave everything else stock.
Installed the chipset drivers? I cant imagine that causing it but worth installing.
If bios as mentioned by someone else maybe resolved in next bios update - bit of a pain to have to wait tho