The reason that ffmpeg won't let you mux the truehd stream into wav is because wav is a container for uncompressed PCM data, not for truehd data. You can e.g. mux it into Matroska. Just use a file with a "mka" extension and ffmpeg will know that you want to mux into Matroska.
Also keep in mind that if you intend to open a ticket in ffmpeg's bugtracker, you need to provide the truehd track with the atmos extension (or at least a snippet of it that allows to reproduce the issue).
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