Aha, now I remembered that when I first run HWiNFO, it asked me that there might some safety reasons to ignore some sensor (it was ASUS G75/X75 EC, but it did not prompt that it is related to fans), so I chose to continue without monitoring this sensor. So no bug at all (though I wonder why...
Hi, in one of the previous versions of HWiNFO it was stated that fan speed monitoring was added for ASUS G75VW. I have this notebook, and I see no info on fan speeds, neither CPU fan nor GPU fan. I use HWiNFO64, version 5.60-3280, under Windows 10 Home.