I have noticed a problem which seems to be in ffmpeg, so I don't know if you can do anything, but here it is... (I used version 3.21.0 on macOS 10.15.4 for the tests)

I have a file I want to cut. So I have set the cutting point to a keyframe, as can be seen here:

When I use the "normal" cutting mode, the sound starts at the right time, but the video is blank until the next keyframe.
When I use the "keyframe" cutting mode, everything starts at the previous keyframe, adding a part of the commercial I want to remove, as if I had selected at this point:

In both cases, the command uses the same time as a stating point:

• normal mode:

ffmpeg -hide_banner -i '/Users/cram/Movies/Handbrake/Opération jupons.mp4' -ss '603.19755' -t '6922.98914' -c copy -map '0:0' -map '0:1' -map_metadata 0 -movflags use_metadata_tags -ignore_unknown -f mp4 -y '/Users/cram/Movies/Handbrake/Opération jupons-00.10.03.197-02.05.26.186.mp4'

• keyframe mode:

ffmpeg -hide_banner -ss '603.19755' -i '/Users/cram/Movies/Handbrake/Opération jupons.mp4' -t '6922.98914' -avoid_negative_ts make_zero -c copy -map '0:0' -map '0:1' -map_metadata 0 -movflags use_metadata_tags -ignore_unknown -f mp4 -y '/Users/cram/Movies/Handbrake/Opération jupons-00.10.03.197-02.05.26.186.mp4'

To support your analysis, I have prepared a zip file including the video from the beginning and both cut files: https://www.jottacloud.com/s/13733169b83ae2749a2a2924a699121bf92