This also happens to me, only way to listen to music with tv off is to set it to autoplay and just turning on the wdep, not turning the tv. This is rubish as i can’t select what i want to ear. May this be some hdmi issue with tv or av receiver?
I think they just implemented the firmware like that: if hdmi power off then go to standby. An update could solve this easily (e.g. new setting in a menu).