Can you download CPU-Z and check the clocks on each core (right click on "core speed") while you are benchmarking? Please test it a few times tell me the results. 

Uh, clocks changed every time and I'm not sure it's a good option to check the clocks, because stress test, temp rises up, and clock speed may getting slower every second. 
I'm using AMD Devtool, and this is the screenshot, the second chart just cropped from the second screenshot. So, CineBench runs for approx. 85 seconds. Because of thread from the same core = same clock, only 4 graphs here (from the graph, up to down, core 3, core 4, core 1, core 2).
1st and 2nd core processor P-State changed from 1 to 2 and mostly 2 every time, that's mean minimum performance while 3rd and 4th at 0 states, maximum performance/ boost possible.
Only getting 467cb for this full load and 100% to CineBench, high-performance and plugged in as always, last 2 days, my maximum 543cb and sometimes downed to around 450.
A lot of internet example suggest 510 is min, and many got 600+ score.
https://www.notebookcheck.net/AMD-Ryzen-5-2500U-SoC.258646.0.html (and youtube test, not gonna drop links here)
My maximum score is 584 back then, but mostly not reach 550, and always more than 500 ofc. That time I just use high performance, plugged in, make sure no background using CPU, etc. Not yet updating anything for days. After tried to update, still no problem till 2+ weeks later.

Well, sounds weird already, someone replaced his new Acer Aspire 3 for defective USB parts with new one, then surprisingly he got a high score from 500-ish to 600-ish (620 actually). While others (at least 2) people asking why he got such high score while they only got 500-ish.
https://www.facebook.com/groups/KomunitasAMDIndonesia/permalink/2165724843709436/
Every product must not in same quality I guess or something like that.
Although, my OpenGL score still higher than his, 40 vs 35 fps.
My 2500u TDP is 15W, not getting any near 15W when at full load with monitoring software like Aida64 and HWMonitorPro, only getting max a little bit higher than 13W, not even touch 14W.
15W from CPU-Z and Aida64.
(This AMDuProf got issue calculating power usage, Aida64 admin already discussed with AMD that this official tool, sadly, inaccurate for power monitoring)
Got some temp problem I think, hotter than usual and airflow from exhaust doesn't even feel normal, not enough heat sucked out? Or maybe heat-pipe or thermal paste isn't good or something.
I think I'll take my Laptop to the service center, but I feel weird if I go to the service center just for this kind of problem. 

Your first graph before cold boot still better, clock rate managed to stay stable for few seconds.
Mine both before or after cold boot, clockspeed graph immediately dispersed.
And I think this happen from beginning, the only difference is higher clock rates, if I recall it correctly.
Core P-State level (frequently scaling, variable frequency or something like that according to internet) for 1st and 2nd core stay at level 2 (minimim, sometimes at level 1), while the rest 0 (maximum), I have no idea this is a feature to controll core speed or just some terms about core speed at low, medium or high speed.

JohnnyK said:

I don't know exactly why and when it happens, but that's the minimal uptime I discovered until it became faulty (sometimes it takes >30mins).

So, your problem still not yet solved I guess. No one wants to pay bucks just to get maximum performance or optimal product quality just for a few minutes. Hardware is not supposed to get faulty just because of uptime.

JohnnyK said:

You could also try to lower the "maximum cpu performance" the advanced power settings (for the Highend powerplan) to i.e. 50%, 75% and 95% and check how the clockrates change under load. I find this odd that mine are stuck at 1.6 GHz on 1-99%, only 100% gives the full boost.

Maybe because CPU fixed speed only can be set to 1.6 GHz, 2 GHz (100%, maximum, base clock, could be boosted). So, if you set max 0-99%, will be stuck at 1.6GHz
I sometimes change the maximum CPU rate to 50% or 99% to release some power to GPU when playing heavy graphics games, because of only 15 default TDP. GPU clock will more stable and stay at the high clock rate.
This may reduce game performance if they mostly rely on CPU. But mostly, my games not even hit 100% load when limiting 50% or 99% maximum CPU state.
Actually if you set X as your max CPU value in power options, 0-50% ≤ X% < ??-99% will result to max 1.6 GHz CPU clock rate and 50-??% < X% ≤ 99% will result to max 1.7 GHz (well, I'm not wanna to check one by one %)
I couldn't see any GPU performance difference when limiting to 50% or 99%, and not even want to take the test to Unigine Heaven 4.0 for it.
Because of more 0.1 GHz could get a bit faster loading times in games, and almost no GPU performance difference.
(Vega 8 iGPU ofc) No problem and no need to limit CPU to achieve even always at maximum GPU clock if your use R3 2200U + Vega 3. And more problem and not worth money if you pick R7 2700U (although overall performance still slightly increased, and decently increased if only CPU or GPU "focused"). Same 15W default TDP.

A feature gone from Power Options after updating windows or something, it's called Core Parking, 1-2 or 3 core could be parked/ idle/ sleep to improve battery saving or could be an alternative solution to improve GPU performance especially when running games that rely only on to the single core. I could hit 142cb back then, single core (I'm sure max at 3,5 or near 3,6 maximum specification, not like now 130+ max 3,4 or 3,45GHz or maybe because I'm testing it with no core parked). And now gone, I also couldn't execute powercfg feature. Maybe the new windows version, new problem. But yeah, this faulty CPU clock happen from the beginning (fresh laptop, after updating bios or other stuff), nothing to do with core parking. I update because of some reason (include that faulty CPU clock rate, flashing when playing video in Chrome, some graphical error looks like artifact but only to page, already clean Chrome, add-ons, update etc no difference, no problem with video software), but well no changes and I feel want to go back.
Using additional software like CPU core parking manager v3, no effect, still all cores enabled, unlike at the beginning, using windows feature also works, not it's gone.

A lot of people getting this problem I guess. As I said multiple times, that new replaced product case (from a bit more than 500 to 620cb), other people with same laptop and spec only getting a mediocre score, and so many references around internet suggest 510 is for super thin and super lightweight laptop, score rises to 550 then up to 620, and there you got 690. Misconfigured laptop manufacturing or something.

Well, I guess I'll ask this further to Acer center to make sure what happens.
Nice Acer product, satisfied, but I just want to get optimal performance as it should be if possible.

And this results after taking cold boot again. 493cb.

Well, it seems will take more than half of month.
10 Nov in service, 5 business days estimate done by 16 Nov.
It's seems software, entire hardware checking didn't solve the problem.
And they shipped a motherboard replacement (include processor ofc, one of "unremovable" component because safety issues) arrived 16 Nov and will be sent from my country capital city to the service point on approx. 20 Nov then dditional 5 business days (estimate), will be done this late November, 27.

I literally told em everything about processor problem include several reference like what I mentioned in this thread. Well a bit surprised it was really a significant problem so they need to ship a motherboard lol.