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How to add black bars to mkv/m2ts dolby vision video files without losing quality?

Is there a command with ffmpeg to add black bars to a cropped 4k Dolby Vision video file? Can be mkv or m2ts or ts doesnt matter.

So what happens if we get a Dolby Vision m2ts with cropped out black bars and they become grey on an Oppo 203 for example. Any way to fix this with tsmuxer or makemkv or sth?

I've converted a few Dolby Vision mkvs to m2ts and it worked. But now more and more dont work and produce gray bars. Why is that? I've tried a few tsmuxer versions but it doesnt work.
What could be the prob here?

I am playing the m2ts files with a Oppo 203.