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I know, 7.1 streams are currently a problem. This is, because ffmpeg does currently not support >6 channels in E-AC-3 audio encoder.
See also https://github.com/HandBrake/HandBrake/issues/1085 and https://trac.ffmpeg.org/ticket/3595
The only option (on Mac) is to use the AAC encoder, which supports 7.1 channels (if your TrueHD stream has Dolby Atmos information, I'm not sure if conversion to AAC will keep this information, but I don't think so).