When converting a movie like, say, Harry Potter and the Deathly Hallows Part 2 from MKV to MP4, how is it you tend to receive two (or three, depending on your preferred configurations) additional corrupt audio tracks coupled with the regular six channels? ???
Please elaborate, especially please provide the textual output of MediaInfo for the source MKV file and the resulting MP4 from reproducing the issue you described using the latest Avidemux 64-bit nightly.
https://avidemux.org/nightly/
I tried to export the single short sample of a MKV file with HEVC as video and a 8ch DTS XLL audio track I have to MP4 and it worked just fine, producing a file playable with mpv and Avidemux with all 8 channels (QuickTime ignored the audio track, but well, it is QuickTime).