Update, an Asus rep sent the 9001 bios update that others have been using, and as soon as installed, CPU temps are reporting what they should be (34c Idle and 53c Load).
Hopefully they can roll this into an official release soon enough so others won't end up with the same issue.
No worries, I'm talking with Asus to see if they can help out.
In the meantime I've just offset the CPU fan curve by about 20c, so that at least the fan is responding to something closer to the real temperature.
Thanks for the clarity Martin, definitely appreciated.
So other people using the Strix x370 board that have reported the same issue, have been able to resolve that issue by disabling Sense MI Skew (using a modified bios that was supplied by someone to add the feature...
So I've just built a new rig, with a Ryzen 1700 and Asus Strix x370. Running HWiNFO64, at fully stock settings it's showing my my idle cpu temp is sitting at 20c (and can even go as low as 16c). Under full load it only peaks at 40c. That seems a lot cooler than it should be running (especially...