How old is the Asus monitor? This might also be a hardware problem, bad caps related. Digital equipment is sensitive to power voltage fluctuations, and when bad caps are in the picture, even more so, making the equpment do all sorts of inexplainable things, like how could one thing I do on this monitor reflect on what the other monitor does or doesn’t. In most cases, a small ground loop or a fluctuation caused by one of the monitors draining power when being turned on or off, might affect what the other one does or doesn’t, if it alredy has failing caps. I’ve seen similar things happen on dual monitor setups when one of them has failing caps. One turns on just fine the other one doesn’t, but you power them in reverse order, hey they work 😂.