mkv losing sound with --audio-source

[fzold]

If you add an audio stream with --audio-source then the mkv muxer doesn't work properly.
The mkv stream has no sound or the sound comes after some time. Something is going wrong with the header.
Simple remuxing with MKVToolNix doesn't change anything. But if you make a new mkv remux, replacing the audio stream the mkv stream has sound.

NVEncC64.exe -i input.mkv ^
--avhw -c hevc --profile main10 -u p5 --qvbr 25 ^
--gop-len 48 --bframes 3 --bref-mode middle --lookahead 20 ^
--audio-source "audio.ac3":copy ^
--sub-source "forced.srt":metadata=title="Forced" ^
-o movie.mkv

Ok, after some tests I found:
Without the --sub-source line it works! The sound is there.

The mp4 muxer doesn't have this issue.