Hey, just created an account to reply here and bring my experience with the 12900k !
First, my professional experience :
I work at the IT dep of a company that need a lot of compute power.
Recently, we built 5 custom workstation for our 3D Cloud point dep. We used 5 12900k on my recommendation. All cooled with a 360 AIO coller (MSI MEG C360)
What I can say with those CPU after 4 weeks of work (and a bit of gaming in our breaks but don't tell our boss

) :
CPU temp is stable at idle. (30°C)
On workload, some "spicks" can be seen around 85 to 90 for P core, but an avg of 80. 100% CPU use
On "Gaming", 65 to 70 avg BUT yes, we have spikes that reach 85... On the 5 Workstations. ~25-50% CPU use
Now what I can say about my personal experience :
With a custom loop, one 360 Rad, Velocity² cpu waterblock and around 0.98L (very good thermal inertia) of coolant only for the cpu, positive pressure case, good fan on every intakes...
I get 25C on long idle and 29-30 on short idle... nice
Ureal Engine 5 compilation (cpu at 100% for 30 min on big project) : little spikes, 70C avg... nice
Intel stress test : 80C... ok cool
Now gaming (on 4K monitor) :
MSF2020 : spikes 85C, avg 70 | DCS world : spikes 83C avg 73 | ready or not : spikes 85C avg 75| my game : spikes 80C avg 69
mmmmh, why does all the 6 CPU have those spikes ?! Here are my answers
1- The GPU... actually when I was playing, my GPU fans were not turning fast... but sometimes they kick off and go to 100%. Even with a really really good front to rear airflow in your case, you will have always hot air that goes to your radiator. (assuming that he is in the top of your case)
2- 12Gen CPU are spiking. I was really scared first because that's the first time that a cpu does it So much

, but "new" tech, new problems and I check on all the internet to see that 95% of the 12900k owner have spikes

3- Thermal paste quality ? Actualy, we used 2 different paste in our build, but nothing changed so...
My solutions : (if you think that you really have a problem)
Change the radiator place, put it in an intake of your case (but don't forget to always keep your pump as low as possible in your loop).
Blow air as much as possible from the front to the rear of the case.
Lower your cpu Voltage, intel extreme tuning software is realy cool for that.
Touch with your hand the inlet and outlet tube of your AIO and if there is no difference, then the radiator might be at fault.
Sry if I misspelled some word, I'm not very good in Eng writing.. I feel like I write like a child