Yes, now the values calculated by Avidemux and ffprobe match, thanks (the same logic should apply to the mp4 demuxer, shouldn't it?).
The 1000 fps issue is a different thing. I was wrong attributing it to the copy mode, it is enough to specify MKV as muxer. The bad: it is not enough to remux the video as mp4 to restore the old fps, both Avidemux and ffmpeg calculate different, deviating values. My test sample: a video with the vimeo-id 67995158
(1080p at 25fps) A supercell near Booker, Texas
- Load the sample video, check the properties
- Save in copy mode or reencode with codecs compatible with the MP4 muxer as a MKV
- Load the resulting .mkv, check the properties
- Save this MKV in copy mode as MP4
- Load the resulting .mp4, check the properties
- Run ffprobe with the .mp4 as argument
The fps value is displayed as 1000.000
fps in Step 3, as 24.991
fps in Step 5 and as 25.03
in Step 6.