I have Q80T and had this same problem. Everything was being downmixed to stereo using eARC, even the built-in apps like Amazon Prime Video. For a workaround, I switched the TV audio output from "TV Speaker" to "Optical" and it started allowing surround sound content . I'm not using the TV speaker or the optical out, but now the multi channel signal is being sent over eARC. Maybe this will help someone else.
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