Patch via https://github.com/HandBrake/HandBrake/commit/8a0847806d558b252daceb5610d2ba61f7e9ba5a

you can test it with matrixbench trailer:

bin/cin /home/admin/K38/files/home/HUAWEI/home/matrixbench_highdivx_ac3.avi

and set encoder to FFMPEG webm - libopus (and video encoder of your choice).

For some reason 4 ch. still does not work, but those files must be rare (made one by accident, trying to encode 2 stereo tracks avi into mp4 without any  audio track disabling)