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The output device is stereo, so I think it's a downmixing issue.

out.dts here's a 5 second-long sample that I can replicate this with.

I'm running Gentoo Linux.

pulseaudio --version returns:

pulseaudio --version
pulseaudio 13.0

pipewire --version returns:

pipewire
Compiled with libpipewire 0.3.10
Linked with libpipewire 0.3.10

mpv --version returns:

mpv 0.32.0 Copyright © 2000-2020 mpv/MPlayer/mplayer2 projects
 built on UNKNOWN
ffmpeg library versions:
   libavutil       56.51.100
   libavcodec      58.91.100
   libavformat     58.45.100
   libswscale      5.7.100
   libavfilter     7.85.100
   libswresample   3.7.100
ffmpeg version: 4.3.1