I'm thinking about upgrading my PC and i have heard about problems in this game using 8 / 12 / or 16 core AMD CPUs.
Is this problem already fixed ?

I like to purchase 5800X and B550

I remember like 1 person complaining about that exact thing, It was hard to tell if poorly configured machine or poorly built and maintained, The user was vague on it kind of like when someone says "hey my machine crashes only on this software I've tried everything" Instantly like magic I know they're lying so IDK, In the thread someone had like a 2600 something which was trouncing the 5800x in performance.

I was looking at getting the same CPU bit uncertain though, Currently on the old 6700k with a 1080Ti, Really want to get a RX 6800 XT but stock what stock and scalpers prices are absurd.

Why the B550 ?

I use an AMD 8 core and the game uses 8 cores without any problem. That means Windows will show something like 50% CPU use.
If I remember correctly someone just wanted the game to use 16 threads, so that Windows shows 100% CPU use. But that is not how the game works and it is not a bug.

I was hoping for a X570 only to get the Gen 4 PCI-E supposed bandwidth increase benefit on a newer card maybe.

B550 also Pcie 4.0 support.

But only 1 PCIe, which is enough for 1 GPU, and 1 M.2 via CPU. And mind some of very cheap B550 do not have M.2 via CPU.

B550 itself is PCIe 3.0. No any problems. Not all threads will be utilized, but 8 threads is enough for CB2077.
6-core AMD Zen do utilize SMT since version 1.05.

Have a 8 threads cpu and the fps go down in some areas of the game to 25 and in 1 area to 17. The game need CORES ;-)

I'm running a 3900X 12 core without any problems.

If 8-thread CPU is AMD FX it's just too slow. It has IPC twice slower than Skylake. I have 3700X with no FPS drops. That's more than enough for this game.

I have 3 m.2 Nvme Drives so Def would go X570, I keep assuming people will pick faster drives etc they make a hell of a difference.

I have a 3600 (6 core) and it seems to use my cores just fine. In fact, someone had said they were using a 5800X and getting worse performance with a better GPU than my 5700 XT. That might have been who was referenced above.

I can run the game on ultra no problems, but it can and does go below 60 fps if I do so.

It's always the case when Nvidia Poisons the well, They simply can't play fairly and cheat and lie to keep above others, They started doing this long ago to Prevent other company's that build to spec for DX and Open standards, Nvidia get an In at the studio of the game say they're assisting or some BS marketing thing and the games run really poorly for other vendors yet these other cards perform extremely well on the same engines under different renderers.

I have cards from all sides and Nvidia cripple things for revenue and nobody is calling them out for it, Shills shutdown convo's and it goes on ignored.

So many angles are being played for Game marketing and releases etc that cause financial shifts.

3700X is a nice cpu with 8 cores / 16 threads. I have i7 4790k and this guy is way to slow for this game. I do not know WHY they recommend i7-4790 (non k) for this game. Hoax.

I have a 5800x and haven't noticed any problems. I read that some hack can improve performance a bit, but i can't complain about the performance much, so haven't tried it.
If you get a 5800x, make sure to get a good cooler aswell. It is certainly on the hotter side.