I'm pretty sure this is a bug in #kdenlive, where the final render of 4K UHD video will cause audio to go out of sync. A workaround is to export video and audio separately, with using proxy clips checked for audio, and then stitch them together with the following command --
ffmpeg -i video.mp4 -i audio.wav -vcodec copy -acodec libmp3lame output.mp4
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