Thank you very much for all your help so far! This is an amazing community.
In the past days, I investigated the problem further and can say that all my playing around and changing the governor, enabling thermald, installing ThinkFan did not change anything. Btw, I already had the latest BIOS version installed.
Instead I was able to find other people with exactly the same problem using other distros: https://bbs.archlinux.org/viewtopic.php?id=246823
My dmesg output is the same:
[ 1.719559] ACPI BIOS Error (bug): AE_AML_BUFFER_LIMIT, Field [TBF3] at bit offset/length 262144/32768 exceeds size of target Buffer (262144 bits) (20190816/dsopcode-198)
[ 1.720455] ACPI Error: Aborting method _SB.PCI0.GFX0.GETB due to previous error (AE_AML_BUFFER_LIMIT) (20190816/psparse-529)
[ 1.721140] ACPI Error: Aborting method _SB.PCI0.GFX0.ATRM due to previous error (AE_AML_BUFFER_LIMIT) (20190816/psparse-529)
[ 1.721834] failed to evaluate ATRM got AE_AML_BUFFER_LIMIT
Interestingly, I was able to reproduce the problem in Windows 10: I uninstalled all Radeon drivers (both the Microsoft and the AMD ones) and suddenly the fan kept working non-stop like in Clear Linux and CPU temperatures raised to about 50 degrees Celsius. After reinstalling the Microsoft driver the fan stopped and returned to normal behavior and normal temperatures (about 40 degrees Celsius).
So the conclusion for me is that it must be a driver problem. And as long as the amdgpu Linux driver is not fixed, the fan will keep rotating non-stop under any Linux distribution. This makes Linux (and unfortunately Clear Linux) unusable for me as the constant fan noise is barely tolerable. In Windows 10 the driver works correctly and there is almost no fan rotating and CPU temperatures are about 40 degrees Celsius.