I also have problem getting audio output device setting stick in Kodi as libreELEC does not seem to recognize my AVR when it's in standby mode.
My setup is as follow:
HDMI 1 Sony XH95
HDMI 2 Pioneer VSX-520 AVR
I am also doing some tests with my hdmi 1.4 receiver like yours and a LG oled 65c8, but until now I have not been able to get the audio from the receiver to work, in your tests the audio from the receiver you have managed to make it work.