Do you have a smart remote in the theater room? You could program a Wattbox or something similar with the remote to come on when you turn on the room and trigger the ET3 on.
I only use trigger 1, trigger 2 has everything off.
If I set output to Monitor 1 or Auto, voltage is 0. It's only when I change output to Monitor 2 that the voltage goes to 12.
Marantz SR8015
The only setting I have on is "HDMI Monitor 2" to trigger a screen.
Trigger voltage goes from 0 to 12.05 and holds there when Monitor 2 is on.
You have two triggers on that receiver. I would switch to trigger 2 and then go in to Settings -> General -> Trigger 2 and select when the trigger turns on. I would also go to Trigger 1 to check if maybe the "on" trigger changed for some reason.