I have the same issue, just tried leaving the monitors on without any an audio cable plugged in. Have also never seen the indicator LED turn red, or turn off. Does not matter how long they are left, standby never comes on.
Actually tried leaving them with auto standby disabled, to see if maybe the switch is just inverted.
They sound excellent and I don't think I would return mine, however it is a shame that the standby feature is missing. A little concerned about unnecessary extra power draw in the long term..
It's strange that both units have this behaviour. Might be best to just run extensions to them and use a unified power switch for both monitors, or something like that.