Sounds like a real headache you've got with your RPi and LibreELEC. I had a similar issue once with a media player on my old laptop. It was super frustrating. In my case, it turned out to be a memory thing – my laptop just couldn't handle the newer software.
You might want to check if your Raspberry Pi needs more memory allocated to the GPU. That sometimes helps with video playback problems. And keep an eye out for any updates or patches for LibreELEC that might fix this. These tech issues can be super specific to the combo of hardware and software you're using.
If you're stuck and need a plan B for video stuff on a Mac, maybe give https://www.movavi.com/mac-video-editor/ a look. It’s pretty easy to use and might help you avoid these sorts of problems.