I have a Ryzen 7 3700x that runs HoTS on ultra just fine (temp 55-70 Celsius) usually, depending on team fights or map density but I am always getting the CPU overheat warning. I have a feeling this is a glitch because the 3000-series Ryzens are super new. It is really annoying having that icon up there. And yes, I am running the correct Ryzen-specific power plan on windows. And yes I have stress tested using Heavy Load and it won’t go over 76 at 100% usage. My GFX is an AMD 5700 XT.
This is a known bug and it will be resolved in an upcoming patch. We have a larger thread discussing the issue here.
Thanks for reporting it!