The same was happening to me on a Zephyrus G15, and it seems that the Optimized power saving mode of Armoury Crate was causing this strange behavior. I.e. if you start the laptop on battery, the Nvidia GPU is disabled. It won't be enabled even if you start something that would require it. However if you start the laptop on AC power, the GPU is enabled and stays enabled if you disconnect the laptop from the AC power while something is using it.