My favorite mp4 metadata editor, mp3tag.de is free for the Windows version but it is not free for the macOS version. I usually just name the filename correctly so that plex.tv is able to read it and fetch the thumbnails for me correctly.
To remove all metadata of mp4 in macOS, I had to use ffmpeg to remove the metadata. The command below is to remove the metadata
ffmpeg -i in.mp4 -map_metadata -1 -c:v copy -c:a copy out.mp4If you do not have ffmpeg in your macOS, you will need to install homebrew (brew.sh) and then install ffmpeg
brew install ffmpeg