video – Use ffmpeg to convert from h264 mpa to h264 mkv

I’m trying these commands:

ffmpeg -i input.mp4 -vcodec copy -acodec copy output.mkv

ffmpeg -i input.mp4 -c:v libx264 -acodec copy output.mkv

but I get the sound only, no picture. On the same input file, VideoReDo 5 identifies the input file as h264 mp4 and saves the file well to h264 mkv with both audio and video. I am a user of these tools but know nothing to the internals of them and video standards. Does someone know whats going on?

For information, my main problem is to reduce the bitrate of the video in the input file to save space on disk.

ffmpeg -i input.mp4 -b:v 850k output.mp4

Neither ffmpeg nor XMedia Recode (which uses ffmpeg) read the file, whereas once VideoReDo has saved it to mkv in a minute (1 hour video) without crying, then only, XMedia Recode or the following command are successful:

ffmpeg -i input.mkv -b:v 850k output.mp4

No update of ffmpeg or XMedia Recode helps, whereas VideoRedo 5 which I got 10 years ago does the conversion job everything depends on (but no bitrate manipulation is available with this tool as far as I know). What is the magic in VideoReDo, to make it do by ffmpeg too? What is the bad magic in h264 mp4? Thanks for any help.