Turns out the AC adapter won't hold 19v at low load (either standby or off) and the voltage just fades away. Even on standby it now it will eventually turn off, and in that state I can't turn on the monitor without unplug/ replug the AC from the adapter. It does or did spike and reset the Adapter as I even saw it come back on, like full restart the monitor as it does with the logo at first boot which I guess is what was happening when I was first turning it off manually.
Now running the adapter on my laptop and it has the same issues coming out of hibernation the adapter doesn't kick in until I unplug / replug the AC, and the laptop adapter on the monitor has been working fine, hasn't dropped out of standby yet. Both adapters are 19v/ 4.74a 90w with the same 5.5/ 2.5 dc plug.
Will try and get the AC adapter exchanged early next week, as it clearly has an issue, why it has the issue is all that's of concern now, as the symptoms are not isolated. Although the Asus adapter does allow the monitor to function when you do get it turned on, I'll put it out there, could this possibly be contributing to some of the other issues being reported ?