Had similar problem in my time with USB disk hooked to the RPI directly and HDMI connection through AVR. Music playback HDMI signal (drops) - #8 by shamael . The issue was only for mp3/flac playback never during movies.
My setup changed a lot so not able to test it back in same conditions but it remained unsolved in its time for me. I ended with a samba share on the RPI and the AVR reading it directly.