Taichi+1700x (almost) no core boost

My rig:
- Asrock Taichi X370, BIOS 2.0
- Ryzen R7 1700X, stock
- 4x8GB Corsair Vengeance LPX3200 @2666
- Samsung 960 EVO NVMe, Sapphire RX480
 
My problem is that the cpu single-core boost doesn't seem to work as it should. From what I read on other forums, running single-threaded benchmarks should always have one core boosted, even if that core changes dynamically as it seems the cpu (or windows?) doesn't assign a thread to a single core but lets it jump between them.

I tried Cinebench (single core), WinRar bench (single thread), Prime95 with only one worker, cpu-z, old single-threaded apps from the early 2000s (Super Pi, PiFast) - none of these are boosting one of the cores to 3.8 or 3.9GHz.

I know that individual cores do jump occasionally to 3.9, hwinfo64 shows that if i leave it running for a longer time (in the "maximum" column), and by enabling graphs of all 8 core frequencies I can see them hitting 3.9 from time to time, usually once every few minutes, for very short amounts of time (like occasional 'blips' if I set polling interval to 500 or 100ms).

Tried all the windows power plans (Ryzen Balanced included), all that changes is the idle behaviour (Ryzen Balanced and High Perf keeps all cores at 3.5, Balanced allows cores to downclock to 2.0 when idle). I've never seen the nominal frequency of 3.4 - under load it always prefers 3.5. Also I don't remember ever seeing 3.8 in the "maximum" column or graphs in hwinfo. So it seems XFR is always active, which I'm not complaining about.

So, is the above behaviour normal on Taichi? If not, what should I look for? BIOS or windows settings? Maybe some special windows update for ryzen? Maybe a clean install, because this windows install is actually from a Gigabyte AB350 Gaming-3 that killed me for 3 weeks before getting the Taichi?
Thanks in advance.

Thanks for the quick and informative reply. A fresh install is on my mind, but after hearing some not so pleasant tales about the new Win10 "creators update",  first I will have to test it on a spare SSD (which I don't have anymore :D )
Right now I'm running Anniversary version, tried updating to Creators but it failed on me.
As for the overclock, I quit doing it some 10 years ago. Guess I'm too old for it. Not really needing the processing power of this cpu, but I bought it because I love tech, I'm a big AMD fan since K5 times, and I had the money to spare :)