I've tested a few nightly releases and everywhere I have to have my AVR turned on during LE start. If it is only in stand by mode then LE will not detect it sees my IT connected to the AVR instead. If I reboot owing to whatever reason I will not have high DTS options available. My AVR is not very typical because it is coming from SONY MDR-HW700DS 9.1ch Digital Surround Wireless Headphone.
Currently I have LibreELEC-RPi4.arm-9.80-nightly-20201231-b257060.img.gz on Pi4+ HiFiBerry Digi+. I can switch passthrough easily between HiFi and HDMI. I've tested also CEC functions to put more pressure on AVR but it doesn't matter even with dissabled CEC LE sees my TV.