I don't think this is a SOLVED case, but it is your decision...
Admin, can you please explain what is the normal behavior of the CPU governor?
What is the proper behavior of the CPU clock in relation to the CPU load?
My phone was updated to 20171227 by flashing using the PC.
After that I made a factory reset.
Later I updated to 20180124 using OTA and I did NOT made factory reset.
I am sure the reset will not solve the issue, because it is there from the very beginning.
The question is what should be the normal behavior of the phone?
And also a request: if someone please can do the same test and tell me how their phone are controlling the CPU speed.
No seller will replace the phone because of such issue under warranty. They will say the phone is in fact usable.
The case is sadly "solved" because, as i previosly stated, overheating to such degree is definitely an uncommon issue wich, in addition, persist to firmware updates or factory resert. This points towards an uncommon hardware problem wich can't be solved here on the forums.
If your seller refuse to get your device back, unfortunately there's nothing else we can do.
By the way, a normal CPU behaviour doesn't include overheating while in idle.
A normal CPU is supposed to deep sleep when not used, or going at 600Mhz with light load.