Just as tip: could it maybe connected to the CEC issue, that HiMEdia has so well known a long ago? Maybe a Lindy CEC-Less adapter could worth a try.
Otherwise I could not think how this could be happening.? Audio passthrough is a digital, unconverted passthrough of bits as it should operate and flow all the same on any devices. What could here make any difference? I mean it's a "standard" that could be made one way: by the standard, or not at all. So how could this end up for some to have it working properly, and for others to have it not working at all (even if the AVRs are completely different).
So that's why I'm guessing there are -MAYBE- other supplementary HDMI functions (CEC, ARC, Handshake?) concerned in the issue...
Maybe i've mistaken something technically, but that's what I can think of it now. More of a question BTW... :