If LUTs are what you’re primarily interested in working with, I’d recommend first demuxing the video stream out of the MKV file before doing anything else with it. Grab tsmuxer (free), open the file using it, uncheck everything except the video stream (h264 for the 1080p and h265 for the 4K), select “demux” above the destination field, then choose where you want the file to go.

You’ll end up with a video-only stream that some programs might not be able to open, but most things that you can use to apply LUTs should play nicer with that than with a muxed MKV file.

You can also check one of the audio tracks if you want to end up with both the video stream and an audio track as two separate files in the end, which might be useful depending on what you want to do with it in the end.