I have Ubuntu 18.04 installed on ASUS Rog GL702VM as a second OS along with Windows 10. After several days on Ubuntu, I have noticed overheating. It seemed like my fan is not speeding up when the temperature is rising. However, when I am working on Windows 10, FAN is automatically adjusting speed depending on the temperature.
Then I did an experiment: I used Ubuntu for 20 minutes, and after overheating started, I switched to Windows. After that, my fan started adjusting speed.