Oh ok, that makes sense then. Here's an interesting
post about the RPM sensor in Windows. So from the looks of it, if the fan is at 0 RPM it really won't show in HWInfo64 until it ramps up. And from ramping up to zero RPM, it remains shown. This is what I've been experiencing earlier when I tested. I was confused as to why the sensor suddenly showed up and never disappeared until I closed and re-opened HWMonitor.
Since I live in a tropical country, I have way hotter temps than you have plus the fact that this system is already in an SM570. But one weird thing I noticed is this:
When I set the fan to Balance mode, I get this:
Does 3879 RPM at 52C PCH temp seem to be correct? Based on the graph, I don't think so. Based on the graph, I estimate approximately 44% PWM. So if the fan maxes out at around 4000RPM, it should just be running at 1760RPM unless the fan can max out at 1816 RPM (3879 / .44)?!!! There's something wrong here for sure.
Also, what is the difference between the PCH and System temps?