Kdenlive uses ffmpeg under the hood and if ffmpeg cannot determine the length of the clip, we cannot use it. In git master, I have implemented a check for such clips, and a dialog now pops up asking if the user wants to transcode the file.
However currently it leaves the transcoding profile choice to the user among a dozen different profiles, which can be confusing – we should reduce the choices. Usually, a basic MKV remuxing works for these cases. Leaving this issue opened until this is implemented.
This will be included in Kdenlive 21.04. In the meantime, you need to manually remux the files, with a command like:
ffmpeg.exe -i myvideo.mp4 -vcodec copy -acodec copy result.mkv
This will create a copy of the original video without re-encoding and will fix the duration issue.