Just a thought: I use a PC as my content source, driving an AV amp via HDMI (audio only). I had to disable screen standby because that turns off the audio output too. Kind of logical, really even if from a functional point of view it should be possible to put a screen in standby without also suspending the GPU and even less so the audio component(s).
That said, I wouldn't worry too much about this. Very few modern LCD screens are susceptible to burn-in and the amount of power drawn by the backlight is really significant only for laptops running on battery. So until the issue is resolved (or if it turns out that your DAC actually needs the screen to remain on just like my AV amp does) just run a screensaver, possibly just a black screen. And of course you can turn off the screen manually if it's an external. I know, soooo old school - but that way you also save on those kwh of standby consumption which aren't good for anything ;)
