Just a small update.
Having exchanged emails with Asus's support, they've told me to check if the PCH temperature can be read from their AiSuite III application ( bliaaahhh ), which it does.
But, on the other hand, their software is really bad and a main cause of system instability, so I don't even have it installed !
Having a really small hope of Asus taking care of this issue in the end.